Prius, Part 2: Why hybrids beat diesels

The best thing about the Prius is that it achieves its high fuel economy without sacrificing size or performance and, most importantly for global warming, without being a diesel. There seems to be a lot of confusion on this point in the comments section of Part One, so let me elaborate.

Bottom Line: If you care about global warming, don’t buy a diesel car (certainly not in this country), and if you must buy a diesel, only get a new one with a very good particle trap. [Does this mean that Europe’s massive switch to diesel was not good for the climate? In a word,”probably.“]

First, diesel fuel has a considerably higher carbon content than gasoline, so burning a gallon of diesel emits 22.2 pounds of CO2 vs. 19.4 for gasoline (see here). A diesel car with the same mpg as a gasoline car would have considerably higher carbon dioxide emissions per mile. [This is offset one third by the fact that diesel has fewer upstream emissions, which, if I did the math right, takes total life-cycle CO2 emissions from a gallon of diesel to 25.8 pounds vs. 24.2 for gasoline (see here).]

diesel.jpgSecond, and more importantly, we have known for a number of years that black carbon (BC) or small soot particles are a major greenhouse gas — and that diesel engines are a major source of BC. A March 2008 review article published in Nature Geoscience, (subs. req’d, abstract below), “Global and regional climate changes due to black carbon,” found that BC may be as much as 55% as potent in total greenhouse warming as CO2.

In October, the House held “a hearing to examine the climate change and other impacts of black carbon emissions” (testimony and transcript here). Dr. Mark Jacobson, Co-founder and Director of the Atmospheric Energy Program at Stanford University’s Department of Civil and Environmental Engineering, testified directly on how BC emissions significantly reduce the climate benefits from diesel cars (here):

It is generally thought that diesel vehicles obtain better gas mileage and emit less carbon dioxide than equivalent-class gasoline vehicles and, therefore, using more diesel vehicles will address the climate problem. However, this concept ignores the larger emissions of fossil-fuel soot from diesel than gasoline vehicles and the resulting climate effects. It also ignores the fact that the addition of control devices to diesel vehicles to reduce their soot and nitrogen oxide emissions, required to meet California and EPA Tier 2 Permanent Bin emission standards and to address the climate problem of soot, reduces the gas mileage of the diesel vehicles. Finally, it does not consider that, in the United States, the lowest-carbon-emitting vehicles in 2006 were gasoline and gasoline-electric hybrid vehicles, not diesel vehicles [See Table 7 in testimony]. The addition of particle traps to the best diesels sold in 2006 in the U.S. would reduce the standing of the diesels further.

[Note: Jacobson explains “the addition of a trap decreases the mileage, thus increases the carbon dioxide emissions from such vehicles by 3.5-8.5%.”]

Also, the addition of a particle trap to diesel increases the NO2:NO ratio in diesel exhaust increases, exacerbating photochemical smog. Finally, even with a particle trap, diesel vehicles still emit more particles than do gasoline vehicles.

Jacobson provides analysis and figures to show that

when diesel vehicles have 30% better mileage than gasoline vehicles, diesel vehicles emitting particles continuously at a particulate matter emission standard of 40 mg/mi or 80 mg/mi may warm climate more than gasoline vehicles for more than 100 yr for a CO2 lifetime of 30 years…. However, diesel emitting at 10 mg/mi (Tier 2, bins 2-6 emission standard) may
warm climate relative to gasoline for about 10 yr at 30% higher mileage.

However, because no diesel vehicle available in the U.S. in 2006, 2005, or 2004 emitted less CO2 than did the best gasoline vehicle available, the 30% scenario in not applicable to the best available vehicles in the United States. As such, 2006 and earlier diesel vehicles sold in the U.S. all caused more global warming than did the best gasoline cars available, over a 100-year period.

And Jacobson also explains:

when diesel vehicles have 15% better mileage than gasoline vehicles, the diesel vehicles cause more global warming over 100 years, regardless of whether they are emitting fossil-fuel soot at a particulate matter emission standard of 10 milligrams per mile (mg/mi), 40 mg/mi, or 80 mg/mi and regardless of the atmospheric lifetime of carbon dioxide (30 or 50 years). This conclusion applies to diesel vehicles having 0-15% better mileage as well.

I did not see anything in the testimony of the other experts that called into question these conclusions.
To repeat the bottom line: If you care about global warming, don’t buy a diesel car (certainly not in this country), and if you must buy a diesel, only get a new one with a very good particle trap. And the corollary — unless you need a much bigger car, you just can’t beat the Prius for total greenhouse gas emissions in this country.

Here is the abstract of “Global and regional climate changes due to black carbon”:

Black carbon in soot is the dominant absorber of visible solar radiation in the atmosphere. Anthropogenic sources of black carbon, although distributed globally, are most concentrated in the tropics where solar irradiance is highest. Black carbon is often transported over long distances, mixing with other aerosols along the way. The aerosol mix can form transcontinental plumes of atmospheric brown clouds, with vertical extents of 3 to 5 km. Because of the combination of high absorption, a regional distribution roughly aligned with solar irradiance, and the capacity to form widespread atmospheric brown clouds in a mixture with other aerosols, emissions of black carbon are the second strongest contribution to current global warming, after carbon dioxide emissions. In the Himalayan region, solar heating from black carbon at high elevations may be just as important as carbon dioxide in the melting of snowpacks and glaciers. The interception of solar radiation by atmospheric brown clouds leads to dimming at the Earth’s surface with important implications for the hydrological cycle, and the deposition of black carbon darkens snow and ice surfaces, which can contribute to melting, in particular of Arctic sea ice.

49 Responses to Prius, Part 2: Why hybrids beat diesels

  1. Wonhyo says:

    In my environmental car discussions “clean diesel” is most often cited as an alternative to efficient hybrids, so I think Dr. Jacobson’s comment is worth emphasizing:

    “Also, the addition of a particle trap to diesel increases the NO2:NO ratio in diesel exhaust increases, exacerbating photochemical smog. Finally, even with a particle trap, diesel vehicles still emit more particles than do gasoline vehicles.”

    In summary, diesel engines produce more carbon (C) than gasoline engines per unit of net energy produced. Whether this C is released as is, captured by particle traps, or converted to CO2, the net result is always more atmospheric heating than gasoline engines. Thus, “clean diesel” produces more climate warming, more health effects, and more smog than gasoline.

  2. Rainhelt says:

    Sorry but thats just not true. It is not allowed, to compare two propulsion systems with the same fuel consumption. You have to take care of the power to weight ratio, to get comparable systems (because you have to compare vehicles with same driving performance). Thereby you will find massive consumption reductions with diesel-systems (15-20%).

    Modern OEM particle traps are in fact very effecitve, so you particle agumentation ist wrong, too. Combined with SCR-Technology, modern Dieselsystems are as clean as regular systems!


  3. Wonhyo says:

    While the Prius is the most efficient and climate friendly car in production today, the Aptera is poised to take that crown later this year. (Aptera switched from a diesel generator to a gasoline generator, because the diesel did not meet California emissions requirements.)

    The Aptera Typ-1 [sic] has less than half the weight and half the drag coefficient of the Prius, with comparable performance. The plug-in series hybrid version gets 130 mpg on hybrid operation alone and up to 300 mpg for a 120 mile trip with a single full charge. An all-electric version, with 120 mile range, is also available. Aptera started accepting refundable deposits for California orders in September 2007. First deliveries are expected in late 2008, with expanded sales areas to follow.

    Since it’s a 2 seater (with additional space for a baby seat), it may be fairer to compare the Typ-1 to the (no longer produced) Honda Insight. At 130 mpg, the unplugged efficiency is double that of the Insight.

    Rumors have it that Aptera is also working on a 5-seat model for release after the Typ-1.

    It seems the unchallenged reign of the Prius is about to come to an end.

    I would love to see an article on Climate Progress presenting an objective evaluation of the Aptera Typ-1. All the specs above suggest it will, indeed, surpass every other production car in climate friendliness. The only question in my mind is whether the composite materials and process are as environmentally friendly as conventional steel construction.

  4. caerbannog says:

    Around here, diesel costs nearly a dollar per gallon more than does regular unleaded (approx $4.80 vs. approx $3.90). If this holds, then folks won’t be buying diesels to save money. Given that refiners get approximately 10 gallons of diesel vs 20 gallons of gasoline (ballpark estimate) from a barrel of oil, I don’t see diesel becoming cheaper than gasoline any time in the foreseeable future.

    Also, folks should be careful not to compare the Prius (mid-sized vehicle, just a little smaller than a Camry) with mini-subcompact diesels when looking at mileage — the usual “apples to apples” caveat applies here.

  5. Rainhelt says:

    Are you sure about the drag coefficient? As far as I know, the Prius DC amounts to 0,28. 0,14 would be amazing!

    Actually, with SCR (selective catalytic reduction) and particle trap, it is possible to meet requirements with a diesel engine, even in California.

  6. Peter Foley says:

    Diesel is longer chain hydrocarbons =more BTUs per pound then the ‘lighter”gasoline, an equal measure of diesel has more energy density. Secondly the piston internal combustion diesel has a higher compression ratio then a gasoline version thus achieves higher efficiency. Please send the author of this post to a remedial tech school before they post any more “non’ facts. I thought a little global dimming was a good thing?
    All the particle # cited are the max allowed, A warm power train will be well under the posted numbers.
    I liked the 25 year-old semi-tractor on a cold start, Can we see Al Gore’s trail of tears from his next private jet’s take off?
    Can your poster have the honesty to post that a large fraction of the current $4.25 gallon diesel cost is directly caused by the adoption of an ultra-low sulfur content diesel specification to appease tree-huggers?

  7. Peter Foley says:

    P.S. Carbon black is a valuable commodity.

  8. Robert says:

    In terms of the climate it is a pointless debate. To achieve 350ppm (or even 450ppm) we should be trying to give up driving not changing car.

    If in 20 years time we have twice as many vehicles, each using half the gas, what have we achieved? Absolutely nothing, except an increased dependency on fossil fuel.

  9. Shannon says:

    The numbers used by DOT are 19.56 and 22.38, based on 99% combustion. But there are even better numbers at which take engine efficiency and fuel production into account, which make the numbers several pounds higher per gallon. Anyway your point is the same.

  10. Joe says:

    As I have said many times, electric vehicles are typically superior from both an efficiency and a greenhouse gas perspective, especially since they hold the prospect of going to a zero emissions running on zero carbon power.

    Much of the world will go pure electric, but I tend to think we’ll need plug ins for the U.S.

  11. Isn’t black carbon technically a “positive forcing” and not a GHG (which are also positive forcings)?

  12. Earl Killian says:

    Does this mean that Europe’s massive switch to diesel was not good for the climate? In a word,”probably.“

    Does anyone know the MPG improvement of Europe’s diesels over their gasoline vehicles? It would be interesting to compare that to the 30% in Jacobson’s Figure 3b. Using US numbers is no good for this purpose.

    If I read Jacobson correctly, the problems described would not apply to algae biodiesel. Still even that would need a decent investigation to know what all the effects would be.

    I think it is worth quoting Jacobson’s paragraph: “Figure 4 shows that the U.S. would need only about 71,000-122,000 5 MW turbines to power all onroad (light and heavy-duty) vehicles in the U.S. converted to battery-electric vehicles. The word “only” is used since this number is much less than the 300,000 airplanes the U.S. manufactured during World War II. Figure 6 shows that the land area required for wind turbines to provide electricity for batteries to replace all U.S. onroad vehicles is only 0.35-0.6% of the U.S. (with the turbine area touching the ground only 1-2 square kilometers).

  13. caerbannog says:

    Can your poster have the honesty to post that a large fraction of the current $4.25 gallon diesel cost is directly caused by the adoption of an ultra-low sulfur content diesel specification to appease tree-huggers?

    Foley is talking through his hat again.

    There *is* a price premium for ultra-low sulfur diesel, but it isn’t a “large fraction” of the current cost (not by a long-shot).

    According to, there’s currently less than a 10-cent per gallon price premium for the “ultra low sulfur” diesel (vs. the plain vanilla “low sulfur” variety). The US gov’t originally estimated that transitioning to ultra-low sulfur diesel would add 5 to 25 cents per gallon. And the real-world data show that the actual price increases are coming in at the low end of that range.

  14. charlie says:

    The only numbers I’ve seen on MPG in Europe suggest the average car in the UK gets about 25 MPG vs 20 MPG in the US.

    Unclear whether those numbers include SUV and light trucks.

    If you look at diesels on midsize cars, the improvements are pretty marginal (30 MPG highway vs. 35). Most of that is the increased fuel density of diesel, plus the great efficiency. Small diesels can get great MPG — but that is as much a function of the weight of the car than anything else.

    Making diesel fuel more expensive in the US makes me think we got lucky. Moving people from large SUVs that get 10 MPG to midsize cars that get 20 is a great improvement. Hybrids are still to expensive for the most part except for really long mile commuters (if gas moves to $9 a gallon that will change).

    What is bad is diesel is great from interstate trucking — the particulate matter isn’t such an issue there — and I think a lot of our current inflation is part of the increase in trucking/transports costs. Hybrid diesels would be great for city trucking, but that is going to take a long time to switch over without government subsidies (the cost of increased fuel are passed on to consumers)

  15. Another way to think of it is that pure diesels do not have the glide path to a 90% efficient motor that hybrids do. Diesel, in some applications may get you to a temporary local maximum in terms of end-use efficiency but doesn’t take advantage of the much more efficient source-to-wheel efficiency of the Renewable Electron Economy (Yes, I know, shamelessly flogging my terminology). Production of biodiesel is some fraction of 1% efficient in converting sunlight into fuel. Solar panels are upwards of 15% efficient now and getting more efficient.

    What the most common renewable range-extender on PHEVs or EREVs will be is still up for grabs. Maybe a biofueled turbo-diesel, maybe a solid-oxide fuel cell run on biofuels, maybe even a hydrogen fuel cell.

  16. John Mashey says:

    I’ve heard Prof. Jacobson talk at Stanford several times, and he is very sharp. His website includes many good papers, including several House testimonies, and some interesting analysis of reliability of networked windfarms .

  17. David B. Benson says:

    Michael Hoexter — Yes, black carbon, i.e.,”soot”, is a postive forcing but it is not a gas at stp.

  18. Peter Foley says:

    Caerbannog, the other 40 cents a gallon is the EU (and China buying generator fuel to go black-out free during the Olympics) bidding up the price to supply all those diesel cars and trucks since their refineries can’t produce enough ULSD (ultra Low sulfur Diesel). You didn’t wonder how 23% drop in heating oil consumption didn’t lower Diesel prices here in the US? My headgear seems to have better sources then that of some others.
    EIA is famous for the quality of its unbiased data, aren’t they the same organization that says we’re running out of radioactive metals? “Our foolish policies are costing citizens billions with little to show for them,” just isn’t going to be released this week by a bureaucratic agency.
    There might be long term issues with the smaller particles changing rain patterns, but I’m still waiting on the Carbon forced AGW model that works.
    Every mandate that shrinks the economy’s efficiency leaves ever less unallocated funds for luxuries such the latest “green” fad.
    I’d focus just a little more on the man in the mirror, and little less on me.
    I’d suggest CaerBannog relying more on primary sources and less on paid shills no matter how convenient. You could help me settle an old argument, do your natural parents suffer a similar intelligence deficit? or is your’s from nurture?

    Has anyone looked at a map of England or Europe lately, Most EU countries can be biked across in a couple of days by a fat guy, the car market is just a little different.
    Also the outragous fuel tax is by the litre which further favors the purchase of the energy dense diesel,more kilometers per litre for the same tax.

    Any one celebrating the new GM Tahoe hybrid which gets the same MPG in town as a 4 cyl Honda accord (21mpg).

  19. Mark Shapiro says:

    Michael – flog away.

    The world’s economy is electrifying, and should continue. Renewables like solar and wind should replace coal, then oil, and finally gas.

    Thanks for pushing in that direction.

  20. Wonhyo says:

    @Rainhelt – Yes! The Aptera’s Cd is 0,11.

    Prior to Aptera, the lowest production car Cd was the EV1, at 0,19. It was said at the time that the biggest components of aerodynamic drag on the EV1 (besides the body itself) were the exposed windshield wipers and the side mirrors.

    The Aptera eliminates the side mirrors, using rear-view cameras instead. The windshield wipers are flush-mounted. The exterior design of the Aptera was done with computational fluid dynamics. It looks like an airplane with wheels instead of wings.

    It will be interesting to see the competition unfolding between the Aptera, Tesla, and Volt. All are designed as electric with range extension. Tesla and Volt are designed for aesthetics and high performance, basically electric sports cars. Aptera is designed for absolute function and fuel efficiency. It looks like Aptera will be the first to go into mass production. Aptera already has over 2000 reservations, which occupies most of the production scheduled for 2009.

    I’m surprised Toyota is dragging their feet on a plug-in hybrid. I think the fiercest competition in plug-in hybrids will be between Toyota and Aptera. It will be refreshing to see a U.S. car company (Aptera) finally fighting for a leadership position in mass market fuel efficient cars.

  21. caerbannog says:

    Caerbannog, the other 40 cents a gallon is the EU (and China buying generator fuel to go black-out free during the Olympics) bidding up the price to supply all those diesel cars and trucks since their refineries can’t produce enough ULSD (ultra Low sulfur Diesel).

    So greenies are now somehow responsible for China’s appetite for diesel? And does anyone seriously think that China really cares how clean its diesel fuel is?

    Anyway, none of this explains why there’s only about a 10-cent per gallon difference between ULSD and the dirtier stuff. Global supply/demand pressures are the primary reason for high diesel prices — environmental mandates are much less important.

    EIA is famous for the quality of its unbiased data, aren’t they the same organization that says we’re running out of radioactive metals? “Our foolish policies are costing citizens billions with little to show for them,”

    So, do you have a more reliable source of price information? If so, let’s have it.

    You could help me You didn’t wonder how 23% drop in heating oil consumption didn’t lower Diesel prices here in the US?

    Perhaps it’s because the price of diesel is determined by *global* markets, of which the USA home heating oil market is only a small part?

    Perhaps it’s because you get only about 10 gallons of diesel (vs. 20 gallons of gasoline) from a barrel of oil, and that demand for diesel is soaring in Europe, China, and India?

    Europe’s appetite for diesel (vs. gasoline) has actually given US consumers a bit of a break on gasoline prices. Europe has excess gasoline refining capacity (thanks to their appetite for diesel cars over gasoline-powered ones), So European refineries have been helping to supply the US with gasoline. If Europe hadn’t gone so hog-wild over diesels, diesel fuel here would be cheaper and gasoline more expensive.

    My headgear seems to have better sources then that of some others.

    Would that be tinfoil headgear?

    settle an old argument, do your natural parents suffer a similar intelligence deficit? or is your’s from nurture?

    Yep. Tinfoil.

  22. Nathan says:

    Two issues come to mind here when trying to do a comparison and I firstly admit I possess no formal qualifications in this field…

    1. An economical diesel running a high percentage of renewable bio-diesel ( particularly with a modern particle trap ) will always be way ahead of a vehicle running gasoline sourced from crude when looking at overall balance of emissions.

    2. What is the effect of the embodied energy when looking at the overall footprint of a Prius when compared to the far simpler diesel.

  23. Robert says:

    Peter Foley

    “Also the outragous fuel tax is by the litre which further favors the purchase of the energy dense diesel,more kilometers per litre for the same tax.”

    Not true. The rates of duty are shown in this link are are significantly higher for diesel than petrol:

    I think Joe oversimplifies the whole US vs EU diesel thing. Both regions have strict emissions limits, shown here:

    Diesel engines are inherently more efficient than petrol – that’s why they are the engine of choice for commercial users. Historically they have been disliked for use in cars because of the knocking sound and the clouds of black soot, but both these problems have been solved and new models drive, sound and feel just the same as petrol-engined vehicles.

    US consumers have an inbuilt bias against diesel, but that’s their problem. Its not based in logic, just prejudice.

    [JR: Sorry Peter, diesels are dirtier than gasoline cars, especially in GHGs.]

  24. David B. Benson says:

    Off-topic, but every small bit hleps:

    entitled “SAS saves by slowing down”

  25. Wonhyo says:


    Your editorial comment two messages above should be directed at Robert, not Peter.

    Apparently, the “clean diesel” marketing campaign has worked wonders. It’s hard to find anyone who understands that the cleanest of clean diesel is still not as clean as modern gasoline.

    I’m amazed at the folly of some of the latest “clean diesel” research, which aims to convert soot (C) to CO2, literally trading one environmental poison for another.

    I think “clean diesel” deserves another in-depth critique, to counter the relentless marketing behind clean diesel.


  26. Peter Foley says:

    CaerBannog, Never meet your parents? how unfortunate.
    The Chinese in brackets was an aside, I’m sorry I confused you.
    I would hold that “Green” factions got the ULSD standard adopted without through consideration of the changeover costs and the drop in refinery production caused by the new requirements. The Chinese would buy cheaper low sulfur Diesel, but the refineries are now producing mostly ULSD. Switching costs prevent changing back to ante ULSD product mixes.
    My pricing info might be similar to your’s, but it appears I have superior understanding of the pricing forces. What was the historical bias between gas and diesel ante ULSD? How does US refineries running at fraction of capacity fit into to your model? I hope you are heavily trading in the futures market.
    *global* markets= reguritating what I said kind of.
    Your crude oil barellel yields I think are circa 1945, you might want to update info.
    Headgear=hat I’m talking out of that is just a little bit better informed then yours, you have repeatedly demonstrated.
    With further thought, you prove that both nature and nurture are needed for completely erroneous thinking.

    Robert, I checked your link out, I read 48.xx pence /litre for both gas and diesel. ULSDiesel to ULSGasoline

    JR Diesels may produce more particulates, but actually costs per mile often are lower then a gas version. I have no horse in the gas vs. diesel race, let the market decide. Again the ignoring the environmental impacts of the creation of the wealth that is destroyed to achieve increasingly tiny increaments in emissions reduction is foolish. The system entropy is increased to achieve a locally “clean” spot.

    Which GHG? The green house gas that doesn’t work, i.e. C02 , or NO or the particulates needed to lower temps allegedly?

  27. caerbannog says:

    I would hold that “Green” factions got the ULSD standard adopted without through consideration of the changeover costs and the drop in refinery production caused by the new requirements.

    A claim you keep repeating without so much as a shred of evidence to support it. ULSD has been required for all vehicles in the USA as of October, 2006. Now, what was the price of diesel fuel then?

    According to, ULSD fuel cost about 1.82/gallon in Jan 2007. High-sulfur diesel was about $1.62/gallon. And LSD, mandated for all on-road use in the USA since 1993, was about $1.84/gallon then. And a gallon of gasoline averaged about $1.80/gallon in Jan of 2007.

    Now, if refinery bottlenecks due to environmental mandates are responsible for the current cost differential between gasoline and diesel, we should have seen a big spike in the price of diesel vs. gasoline back in 1996, when the ULSD mandate went into effect in the USA, or even earlier, when the ULSD mandate went into effect in Europe.

    But we didn’t. What we do see is prices of gasoline and diesel (both high and low sulfur) steadily increasing in price, with the diesel price increase more rapid in the past few months.

    CaerBannog, Never meet your parents? how unfortunate.

    Oh, I’ve met my parents, and I know they aren’t cousins.

    And BTW, how is your father with the banjo?

  28. Rainhelt says:

    @Wonhyo: We did the same thing with the mirrors in a study last year. Actually, mathamatacally this should lead to an reduction of drag coefficienbt and thereby result in a reduction of consumption. But the effect was pretty small (0,1-0,3 l/km) due to small velocity level in real people driving behaviour.

    It is much more valueable to reduce vehicle weight, to achieve consumption reductions. Thereby we figured out, that full-hybrid-drives still have a bad system weight and by now mild hybrids seems to be the better solution (Like in your example a stareter generator system…)

    I don’t undertand why the autor is still saying, that “diesels are dirtier than gasoline cars, especially in GHGs”. For a standalone engine/concept, it is true. But with modern exhaust aftertreatment systems, that`s just not true. The combination of standart catalytc converter (oxidation), scr (Urea), particle trap and EGR combined with with modern injection systems is really good!

  29. Peter Foley says:

    Caerbannog, I’ll try to teach the pig to sing.
    1. ULSD destroys over 1% of the energy of diesel, thus instantly creating an increase in demand.
    2. The change over wasn’t instantaneous-thus negative effects took time to manifest.
    3. Your info about the time-line of the current ULSD standards is wrong.
    4. The bottlenecks are the EU didn’t rebuild THEIR refineries to produce the fuels THEIR government required and are now buying the shortfall on the the spot market.
    5. The innocent are paying for the EU’s market mess, the latest version of Brussels’ socialism/tyranny.
    6. the added pollution controls on new diesel power has increase fuel usage(incidentally carbon output) thus further lowering supply.

    “steadily increasing in price, with diesel price increase MORE RAPID in the past few months” –the last part agrees with my earlier statement.

    Not cousins, perhaps siblings? hopefully your brothers and sisters don’t suffer your deficits.
    Ask a musician how hard it is to play the banjo, was that meant to be an insult?
    I hope your understanding of climate issues is of a higher caliber then you have shown regarding the oil industry.

  30. Robert says:

    I think all this stuff about diesels being dirty is a pile of crap. If the carbon particles are so dangerous how come average ages in Europe just keep going up and up? Most likely the anti-diesel thing in the US is a form of import barrier to prevent EU cars making headway in the US.

    In any event, the whole subject is boring and has little to do with climate change, which is what this blog is supposed to be about. The fact is that until we find a way to drastically reduce overall global fossil fuel consumption year on year the climate threat will continue to deepen. Arguing over the proportions of crude that are cracked into diesel vs gasoline is just tinkering with the deckchairs.

  31. Greg N says:

    Joe, firstly you Americans need to start thinking in terms of CO2 emissions in grams per kilometre rather than mpg – because mpg doesn’t make much sense in a plug-in hybrid world!

    It takes a while to get acclimatized, but in the UK all car adverts have to quote the g/km CO2 figure.

    For example, the UK car fleet’s average is 164 g/km. And the road tax banding is directly linked to g/km (drive a car with emissions of 119 g/km and you pay an annual tax of £30; drive one with 230 g/km and you will soon pay £430).

  32. Greg N says:

    Secondly, you didn’t compare apples with apples in your piece, which makes your argument flawed. Comparing two cars with the same mpg is pointless.

    Instead, take two identical cars and put in either a petrol or a diesel engine, and compare the results. For example, a Toyota Yaris 5 door with either a 1.3 litre petrol engine or a 1.4 litre diesel.

    Both cars have similar “performance” (i.e. top speeds of 105 mph, reasonable acceleration).

    The petrol emits 141 grams of CO2 per km. The diesel emits 119 g/km. Choosing the diesel has a benefit of 16% for CO2 emissions throughout the car’s lifetime.

    Are you/Jacobson claiming that the diesel’s black carbon cancels out this 16% gain? What is the estimate for the climate “cost” of the particle emissions? You quote a figure for Jacobson’s trap of only a 3.5% to 8.5% increase in emissions, not enough to cancel the 16% benefit. (I’m afraid you/Jacobson didn’t write clearly enough for me to be able to understand the paragraphs about 15% better mileage.)

    [NB this compares with the UK rating for the Prius of 104 g/km. But then in the UK a Toyota Prius costs £20,000, a Yaris diesel £12,000 and a Yaris petrol £11,000. A Prius is a very expensive way to get good fuel economy compared to small European diesel cars, even with petrol/diesel prices at $10 a gallon.]

  33. Greg N says:

    Quick conversion between miles per (US) gallon and CO2 grams per km:

    g/km mpg
    100 54
    120 45
    140 38
    160 34
    180 30
    200 27
    220 25
    240 22

    To be honest, it’s embarrassing to hear figures of 20 mpg for a car. Alternative cars with twice the efficiency/half the CO2 emissions exists to buy right now – petrol cars, diesel cars, hybrid cars.

    A whole wedge, available right now, without a penny needing to be spent on R&D…

  34. steve shoap says:

    The best way to get high mpg is to make cars lighter. Unless you use exotic materials, the only way to make cars lighter is to make them smaller.

    Smaller cars raise the risk of death and injury in accidents.

    Please see my website
    for my ideas on how to make small cars safer.
    If you like what you see, please contact the auto companies.
    At this time, they have rejected my ideas.

  35. Wonhyo says:

    Greg N – I think Romm/Jacobson are, in fact, claiming the soot emission (C) of diesel cancels the 16% advantage in CO2 emission. Soot captures heat not just in the atmosphere, but also when it settles on polar ice, after circulating through the atmosphere. Soot is also a greater immediate health hazard than CO2.

    All of the techniques to make diesel cleaner chip away at diesel’s energy efficiency, but still don’t make diesel as clean as gasoline. If you include the greenhouse effects of C, there is no advantage to diesel.

  36. Wonhyo says:

    If you look at the the differences between gasoline combustion and diesel combustion, it’s easy to see why gasoline is inherently cleaner than diesel, even with all the attempts to clean up diesel.

    In gasoline engines, the air and fuel are mixed in a precise ratio prior to combustion. This ratio is controlled and optimized throughout the operating range of the engine to minimize output of NOx, hydrocarbons, and soot. Thus, the post-combustion cleanup process is relatively simple.

    Diesel engines combust with the same amount of air, but varying amounts of fuel, depending on the operating range – extremely lean burn at idle, slightly lean burn at max power. Furthermore, the fuel is injected AFTER the air is compressed, resulting in incomplete mixing. The outputs of combustion vary dramatically depending on power output. The combustion process and the post-combustion cleanup cannot be efficiently optimized throughout the operating range.

    Every attempt to clean diesel emissions robs it of energy. You can compare engines of equivalent energy efficiency and find diesel is dirtier. You can compare engines of equivalent pollution and find gasoline produces more energy. Either way, the fundamental nature of diesel combustion guarantees that any energy advantage is accompanied by a larger disadvantage in pollution.

  37. Rainhelt says:

    “Smaller cars raise the risk of death and injury in accidents.”

    Are you sure? Do you no the SMART of Daimler? Its one of the smallest cars. And it is safe. It meats all saftey requierments all over the World.
    It is just a question of engineering and price.

  38. Wonhyo says:

    Steve Shoap – Please take a look at They are taking orders for a 1,500 lb composite body plug-in electric hybrid with 130 mpg efficiency before plugging in. (orders currently being taken in CA only, pure EV model also available)

    Aptera is designed to exceed federal safety requirements, using a race-car like roll cage, crumple zones, and a shape that will make the Aptera slide up above an impacting vehicle.

    BTW, the plug-in hybrid Aptera prototype used a diesel generator, but the production model uses gasoline, to meet CA emission standards.

  39. Greg N says:

    The official particulate matter emission for the Toyota Yaris is 0.021 g/km.

    Which translates to 34 mg/mile.

    Jacobson seems to be giving estimates for three levels of PM emissions: 10, 40 and 80 mg/mile.

    So seems sensible to concentrate on his 40 mg/m illustration.

    Unfortunately Jacobson’s graphs and commentary at
    seem intended to confuse rather than illuminate.

    If I interpret it correctly: if the diesel Toyota Yaris was 30% better in CO2 terms than the petrol Yaris then it would reduce global warming long term (though the soot would warm short term).

    But given the diesel Yaris has 16% lower CO2 than the petrol, then as Wonhyo says the soot emission problem exceeds the CO2 advantage on an “all-in” basis.

    I’m open to any corrections on this interpretation!

  40. Wonhyo says:

    Greg N – The following line from Jacobson’s report seems to sum it up: “In sum, there is not an advantage and a potential disadvantage of diesel versus gasoline in terms of climate and air pollution impacts.”

    The reality we’re experiencing supports this conclusion. With a whole bunch of fancy gadgets (SCR, ECR, particulate trap) and additional maintenance (ammonia/urea supply), modern diesel struggles to comply with California’s minimum emission standards, while gasoline cars go far beyond the minimum and achieve ULEV/SULEV status.

    Also, the “short-term” life of soot is in the atmosphere. Once the soot settles on polar ice, it doesn’t just disappear. It continues to convert sunlight to heat.

  41. Robert says:

    The VW Blue Motion brochure makes interesting reading:

    All engines (petrol and diesel) comply with the toughest emission standards currently in force (EURO 4). For diesel engines this is:

    CO 0.50
    HC+NOx 0.30
    NOx 0.25
    PM 0.025

    This last figure is the carbon particles and will pass anywhere in the world except the latest California LEV II standard of 0.01

    What is the Prius like on recycling? If you read the VW Blue Motion ads there is a whole section on how every part is marked with the material type so it can be fully recycled and rare metals recovered from components such as the catalytic converter:

  42. Greg N says:

    It’s not good enough merely to cite California’s standards.

    The entire European strategy for cars and global warming is based around a switch to diesel. By saying California is right, you are saying the European Union is wrong.

    There’s a big tax incentive in the UK, for example, to choose the Yaris diesel over the Yaris petrol – lower annual road tax and lower taxes at the filling station. I would save money with the diesel, despite paying £1,000 extra for the car.

    Half Europe’s new cars are diesel due to this strategy and incentives.

    This is perhaps a bigger story than Joe and Jacobson realize – it’s saying an entire continent is on the wrong track.

  43. Greg N says:

    Right, I’ve finally managed to decipher this article.

    The Toyota Yaris diesel has 32% better mpg than the identical car with a petrol engine.

    However, the CO2 emissions are only 16% better per mile (because burning a litre of diesel emits about 2,630 grams of CO2 compared to 2,300 grams for a litre of petrol).

    The diesel engine also emits 0.021 g/km (34 mg/mile) of particulate matter emission. The global warming disadvantage of this soot counteracts the CO2 advantage.

    With 32% better mpg and 34 mg/mile of PM, this diesel engine is slightly on the better side of Jacobson’s illustration of a 30% better mileage and 40 mg/mile PM car. Figure 3b in his report shows the net impact of the soot disadvantage vs the CO2 advantage for this illustration – very marginal, but a very slight worsening of global warming. The little bit better PM emission of the Toyota Yaris’s 34 mg/mile probably pushes it very marginally onto a very slight improvement side (but to be securely on the improvement side diesel engines will have to achieve only 20 or 10 mg/mile).

    Conclusion: the warming impact of a Yaris car is pretty much identical with a diesel engine or a petrol engine. The headline CO2 saving is cancelled by the soot impact. The big tax incentives offered in European countries to persuade people to buy diesel engines are thus misplaced, but at least aren’t making things worse.

    [NB The Toyota Yaris comparison looks to be fairly typical of small European cars. A Peugeout 207 can have either a diesel engine or a petrol engine: the diesel gets 40% better mpg and 20% better CO2 emissions; its PM emission is 29 mg/mile.]

  44. Ryan says:

    I’ve heard (but not seen written evidence…so point me there if you know!) that the energy used in producing a Prius, the battery, and shipping it/parts to the U.S. uses more energy/CO2 emissions than could possibly be made up for by driving it…no matter how much you drive. Has anyone else heard this? I sort of doubt it given the 2x improvement in mileage per gallon, but I’m not sure. Thanks!

  45. Robert says:

    Ryan – The article below claims that a Prius consumes the equivalent of 1000 gallons of fuel in its manufacture, so you have to drive 100,000 miles or so just to catch up with that second hand car you junked in order to go green…

    This shows tha value of (a) design for recycling, and (b) keeping the design as simple and minimalistic as possible and (c) replacing vehicles less often. The Prius has all those extra components (regenerative braking, electric drive mechanism, batteries, control electronics) which all have to be manufactured, serviced, carried around in the vehicle and disposed of / recycled and which, on a freeway journey, contribute nothing.

  46. Trollhattan says:

    A few notes:

    -Refining methods used in the US produce higher ratio of gasoline:diesel than those used in Europe, so diesel production will always lag in the US and as one result, diesel vehicles will necessarily remain a smaller portion of the US fleet.

    -Without having switched to low-sulfur diesel, we would be unable to use the latest high-efficiency, high-output diesel engine technology (which among other characteristics uses very high fuel pressures unavailable to the older fuel). See, e.g., Audi at LeMans. This ain’t your cousin’s diesel Eff Two-Fiddy.

    -US federal tax on diesel is higher than for gasoline, $0.244/gal vs. $0.184/gal.

    -PM2.5 and PM10 standards will continue to be tightened to reflect the increased understanding of their negative health effects. Regulation is being extended to heavy construction equipment and stationary sources, such as diesel generators.

  47. kris says:

    What about the Boat that brought the Prius over from Japan? I bet the diesel engines on that ship are not of the “high efficiency” variety. Why are those emissions never figured into the footprint of a prius?

  48. kris says:

    Does this article even begin to look at alternative fuels? the diesel will still get 60mpg using Biodiesel. If the prius could use E85, its mileage would drop.

  49. dr61 says:

    Excellent discussion. I believe Jacobson’s conclusions regarding particulate filters reducing efficiency significantly are now out of date. Consider the following data from EPA’s

    2009 VW Jetta 2.0 Diesel (with particulate filter): EPA 30-41 mpg, HP 140, ULEV certified in California.

    2006 VW Jetta 1.9 Diesel (no filter): EPA 30-37 mpg, HP 103, not certified in California.

    There has been an INCREASE in efficiency with the filter-equipped engine for these two vehicles with identical bodies and weight. And the ULEV rating (better than some gasoline cars) was obtained without urea injection. Honda has developed a similar engine that should be available soon.