John Goreham's picture

Why are there no diesel hybrid cars?

We analyze why automakers have not come up with an affordable car using a diesel electric-hybrid powertrain. Seems obvious, right?

One of our readers posted a great comment under a story recently asking why there are no diesel hybrid cars in the US market. At first blush it seems like a no brainier. Both appear to offer great fuel efficiency. Why not marry the two? Maybe appearances are deceiving and there are some sound reasons against the technology, or maybe it is just convention. Let's break it down.

First, we should review the goals of cars like the Prius and Honda Accord hybrid. Obviously achieving the most miles per gallon practical is the first goal. Next many would list environmental friendliness. This goal has a few aspects, but emissions at the most measurable and most noted environmental metric, with CO2 emissions in particular being important. Fuel economy is certainly a goal. This is related to fuel efficiency and MPG, but takes into account the cost of the fuel. This might seem obvious, but lowering the amount of crude oil used to make the car go is an objective of many hybrid buyers. Finally, overall cost of ownership needs to be considered. You might notice we have left off drivability. Yes, everyone wants a car that can accelerate safely, but a quarter million people per year buy a new Prius or one of its copycat makes, and they all are relatively slow and handle a little sloppily. We can include this as a goal, but let's not delude ourselves. Most affordable hybrid buyers put that low on their list of needs and wants as proven by current and past sales.

Now let's get the hard facts out of the way. There is no affordable diesel car in the US market that is the fuel efficiency leader (MPG combined) in its class. We have done that story before so if you need to look back do so now. Gasoline cars are the liquid fuel champs in the US in terms of combined fuel efficiency. So starting with a diesel has that disadvantage already.

The next goal is emissions. We want them to be as low as practical. Here again gasoline cars are beating diesel cars. Until very recently diesel cars were much more polluting by pretty much every measure. Then government stepped in to stop that. Diesels now are not allowed to produce the fine particulate and oxides of nitrogen they used to and need to be in line itch gasoline cars. However, the diesels still produce more CO2 per mile than gasoline cars. So in terms of measurable environmental impact, gasoline cars are again the starting place.

Fuel economy is next. Here in the US diesel costs about 20% more than gasoline. Taxes are only a small part of the reason why. Things could be done about that which might help with this, but in the US we presently use all the diesel fuel that we produce domestically. Sure, we could artificially lower the price of diesel in comparison to gasoline like many European countries do, but why? The fact is that just to match the fuel economy of a gasoline car like the Corolla, which can get 35 MPG combined, a diesel rival needs to achieve 42 MPG.

Related to this issue is that diesel in America is currently much worse than gasoline in terms of miles per gallon of crude. That is because l like it or not, the US refineries create much more gasoline from a gallon of crude than diesel. The EIA says it is 11 gallons of diesel and 19 gallons of gasoline per barrel. Since we are already a net importer of liquid crude, why import more just to switch to diesel?

Diesel vehicles offered by automakers that also have a similarly powered gasoline engine option charge more for the diesel. It seems the diesel engines cost more to make. So the initial cost is higher. Could that be offset by lower maintenance costs, or higher residual value? Maybe.

Finally, we come to drivability. Diesel lovers always cite two things about the cars they like most. First, the relatively higher torque compared to naturally aspirated gasoline cars. To me that is a non-issue. All diesels now use turbos, and the torque is nice, but turbocharged gasoline cars are also torquey. The electric motor drive from the hybrid is also going to add torque to the vehicle, so the diesel advantage here is not very helpful. Plus, there are turbocharged gasoline hybrids on the market now so that is clearly a possibility.

Next the diesel advocates cite the great highway fuel economy of diesels. This is a valid advantage of diesels. Remember though, the hybrid we create is going to have all the aerodynamic and rolling resistance improvements possible. Is slightly better highway mileage worth the compromises we have already listed out that diesels force us to live with?

After a closer look the diesel hybrid makes less sense. If you need any proof that automakers that have in the past pushed diesel are now turning to gasoline electric hybrids look no further than Volkswagen. Their new Golf GTE hybrid will soon join the Jetta (turbo gasoline) hybrid in the US market.

Related Stories:
Why a compressed natural gas plug-in electric hybrid makes sense
Diesel vehicles blamed for polluting Paris air by authorities

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Diesel will not catch fire in a crash. It is the much safer option, which will be front and center in the next car I buy. There is a lot of burns and deaths due to gasoline -- its just too dangerous. For the safety of the children, diesel is the only choice.
This is a darn good point that I've never seen brought up. Thank you.
Glad you brought that up because reducing risk offire and explosion in an accident is more important to me than fuel efficiency. Also, I believe you can safety store a few gallons of diesel without risking an explosion, but not sure about that.
How often do people burn to death in crashes? Or do you still own a Pinto? Death by fire, in auto crashes, is very rare. It's simply not an issue. And diesel most certainly DOES burn or it wouldn't be of much value. It's not as easy to light, but it will light. And once it gets going it keeps going for a long time. Diesel fuel tanks can be MORE explosive than gasoline precisely because diesel is less volatile. A gas tank's atmosphere, above the fuel, quickly reaches the point where the mixture is too rich to burn. Diesel fuel's evaporation is so slow that an "explosive mixture" can last MUCH longer. It can even keep pace with the fuel use as you drive, so you have an explosive atmosphere in the tank ALL THE TIME on a long trip. That's why gasoline powered aircraft have sealed fuel systems but jets are continuously vented to the atmosphere. In short: It's harder to ignite spilled diesel fuel, but the tanks can be MORE explosive in some conditions.
Recently, there was a crash near Basel, switzerland, where VW golf hit and drove over a street signal. The plastic tank was cut open by the rest of the steeltube, setting the car nearly immediately on fire, one person not wearing a belt could -with severe head injuries- because she flew through the windscreen, pull out one passenger (with belt) out of the burning car, several others were burned... That's probably why my Opel Astra Build 1994 has a stainless steel tank... Which is about 1 inch higher then the bottom of the car. Point is no diesel car would burn that fast, sorry to tell! That is by the way the reason why nearly non new tank is gasoline driven, although that means that in cold (arctic) aereas, there must be a sophisticated heating system for the diesel.
OK, well that's another reason why there SHOULD be diesel hybrids....
I think the biggest reason is the price. Making a car a diesel or a hybrid both add thousands to the sticker price...combining them makes it even worse.
What Luke said. :)
Why has diesel fuel been more expensive than gasoline? On-highway diesel fuel prices have been higher than regular gasoline prices almost continuously since September 2004, a break from the historical pattern of diesel fuel prices usually being lower than gasoline prices except in cold winters when demand for heating oil pushed diesel fuel prices higher. The main reasons why diesel fuel prices have been higher than gasoline prices in recent years are: High worldwide demand for diesel fuel and other distillate fuel oils, especially in Europe, China, India, and the United States, and a tight global refining capacity available to meet demand during the period of high economic growth from 2002 to mid-2008. The transition to less polluting, lower-sulfur diesel fuels in the United States affected diesel fuel production and distribution costs. The Federal excise tax for on-highway diesel fuel of 24.4 cents/gallon is 6 cents per gallon higher the gasoline tax." - EIA statement. So WHY does Europe subsidize diesel and US subsidize gasoline? Diesel demand is higher globally...WHY? US refineries are optimized for legacy gasoline car volume...could be optimized for higher mix of diesel fuel. If we start EXPORTING nat gas, that will surely raise that price in US just as Diesel exports (safer, less explosive) drive prices up in US. Can we see real energy balance analysis on which fuel would use less crude oil if demand IN USA was based on 50:50 mix?
You're being over-simplistic and you're making some assumptions that are actually incorrect. Diesel and gasoline can be made from the same barrel of oil. Generally, an oil refinery tower can be seen as a large boiler with strata of heat. The hotter things are, the higher they are in that tower. Diesel is somewhere in the middle while gasoline is below it (cooler) with much of our gasoline coming from a coker system of refining that adds further steps to the process. More gasoline can be extracted per barrel of fuel vs diesel, but less effort is required to make the diesel fuel. In the past, this was generally a wash and the low cost of oil often made diesel cheaper overall. Another thing to remember is that "worldwide demand" for diesel and other fuels doesn't mean anything other than how it affects oil prices themselves. We do not import diesel or gasoline, we make it here. Nearly all other countries do the same - import the oil, refine it yourself. It makes no economic sense to do otherwise. The reality is that the majority of the diesel cost difference at the pump is from two factors: higher state and federal taxes on it (versus gasoline) and higher demand with lower availability due to many areas having bans on building new refineries. California is a prime example of both those paradigms working together to force diesel importation from other states. I know a couple of drivers who make a lot of money hauling diesel fuel from Reno, Nevada to Sacramento due to this. Finally, as you point out, the desulferization process adds cost to the diesel mix. U.S. refineries, however, are not necessarily "optimized for legacy gasoline" as you point out. What's happened is that transportation costs for gasoline have required that at least half a barrel be made into it in order to turn a reasonable profit. This requires 20-30 percent of the oil be cracked or reformed (or both) at the expense of other products in the process. Gasoline is expensive and difficult to transport, hence the cost. Europe chose diesel over gasoline for one simple reason: in the 1970s/80s, it was cheaper, cleaner, and more cost-effective environmentally and economically to go diesel there. Since most European countries import oil or fuels, they had to choose and diesel is more energy dense and easier to refine than is gasoline. The rest of the oil can be used for other things without requiring that gasoline be extracted unless it's as a by-product.
i disagree with your findings on diesel. there was a time when diesels put out a soot mthat went to the ground and didnt cause airborne pollutants. there was also a time when diesel oil was a waste product of refraction. similar to flouride another waste product, wait we put that on our teeth. anyway, there is a lot of developement that reduces diesel's usefulness, then again coal powder was the original fuel for the diesel engine until mr diesel was killed.
Your point doesn't make sense. Not sure what you are saying, because everything you wrote agrees with my point.
i guess great minds think alike
I think the question in US is not about Diesel Hybrid but all Diesel. In Europe they are a lot of Diesel (taxes are more favorable to Diesel) and also Diesel Hybrid. Volvo V60 Plugin Diesel Hybrid, Peugeot 508 RXH, Citroën DS5H... It is not a technical problem only a market difference
Interesting analysis, although the title is somewhat misleading: there are diesel hybrid vehicles, although not in the US. I just wanted to point out something: if you disregard the torque advantage of the Diesel engines because " turbocharged gasoline cars are also torquey", you have to consider the fact that turbocharged gas engines, in general, use Premium gas. Right now the price of Diesel on my area is exactly 2 cents higher than the 93 octane gas, which for all practical purposes is negligible. A Turbo Diesel Hybrid would probably be more advantageous in heavier cars, though.
I get about 40 mpg in my 2011 VW Golf TDI in town. And my city is fairly hilly. Last year I went on a cross country trip and got 60 mpg over the whole trip. I will say I do know how to drive to get the best mileage. The next generation Golf - due out very soon - will get perhaps 10% better mpg than the model I drive. A few thoughts...If the Golf was built as lightweight as the Prius models it would be substantially lighter than it is now. The next-gen Golf is lighter weight but will likely be less tin-cannish than the Prius models. The Golf is a substantial feeling car. There are many that suggest hybrids do not get the mileage that is claimed. In contrast, diesel models tend to get get better mpg than government figures show. Another thought is in regard to torque. We all know the word but, personally, I learned what it was when I bought this Golf TDI. As time goes by more aluminum and carbon fiber will be used allowing cars to be lighter. But the problem for hybrids is although the body and some components can be made lighter the heavy batteries, and motors will remain major weight impediments - and increasingly becoming a higher percentage of the overall vehicle weight. Conversely, diesel engines and transmissions will become smaller and smaller - greatly increasing the already excellent mpg. And if desired diesel cars could be tuned to be as slow as Prius vehicles which would give them even better mpg.
Where do you get your information? Diesel engines produce much less pollution that gasoline cars! All engineering classes I took at University of Wisconsin discussed this and that is why emissions test don't check diesels. wow, you give a lot of misinformation...
Diesels require much more elaborate exhaust after-treatment systems to achieve acceptable emissions...unless the pollution you refer to is carbon dioxide. In that case, you're right.
One thing I never see is the fact that diesel engines on average last two times longer than gas engines. So why is everyone nitpicking the cost difference? Also if the motor last longer (making the car last longer) wouldn't that be more green, and bring the overall cost lower than a gas car?
diesel generators on an electric car is great idea only a small diesel can produce the torque at lower rpms to run a generator capable of pushing multiple electric motors and still supplying comforts of ac and heat and full electric doors windows etc.this formula works for the railroads and makes them the economical leader for large freight moving and TADA Volkswagen has a 300mpg diesel electric hybrid prototype!
Diesel hybrids do make sense, and are being sold in other countries! The only reason I still drive around my >500,000 mile 1985 VW Golf Diesel is because they don't sell any diesel hybrids here yet. I just love it when I compare my MPG in my 1985 Golf Diesel to brand new hybrid owners and get better fuel mileage than they do! As for polution, diesels actually do make less than gasoline cars if you don't have all the emissions stuff on them. Actually cars with dirtier exhausts tend to be the easiest to clean the exhaust on. Take the Mazda RX7 for an example. The HC and CO emissions without the catalytic converter were some of the worst in history. But all that bad exhaust made it easier for the catalytic converter to heat up and do its job, making the RX7 actually get some of the best tailpipe emissions of its time. Nowadays, gasoline engines are actually following diesels on both operation and emissions with modern direct injection and lean burn technology. With diesel hybrids getting into the hundreds of miles per gallon, like the VW XL1, I don't see how they are producing more CO2 than gasoline hybrids.
Why not a natural gas hybrid? The US has lots of natural gas and it burns cleaner.
Hi There, You asked why not Natural Gas hybrids? Which is an excellent question. Even better why isn't he gov't pushing for using NG in all vehicles especially given the abundance of it? It is nearly ZERO on emissions and engines would last a lot longer. The so called Dept of Energy, which has done nothing since its creation to create more efficient use of energy should be running the show to promote NG use. And Obama who is preoccupied with all things Black should be asking Congress to pass legislation to authorize gov't financial support in creating the appropriate infrastructure in creating NG distribution network similar to diesel. This could be started with truck stops which are near NG pipelines and then grow from that. When I purchased my first diesel in 1974 there was a problem getting diesel fuel. Most of the time had to go to a local truck stop. This diesel got 25 MPG when most gasoline powered car were only getting 10 to 15. Since then several car makers have improved those numbers. Unless the DOE gets off of it backsides and earn their keep that agency should be dissolved. That would be one way to lower the annual budget.
There are significant errors in the rational given on why no diesel hybrids. TRUE, the current diesel powered cars sold in the USA today only match the efficiency of gasoline powered cars. However Mercedes offers about 6 versions of a diesel powered car in the UK. Several of which provide phenomenal MPG numbers in the 60+mpg range. So a Google on Mercedes UK and check that website. Plus because of the higher torque in diesel engines their performance is better that gasoline power US cars of the same Cubic Centimeter size. And if that isn't enough check out the VW diesel hybrid which is said to be able to get over 100 mpgs. Yes it is expensive but it illustrates that current diesel hybrid technology car provide higher MPGs. The VW diesel hybrid used all manner of exotic materials to make the car a light as possible. But using less exotic materials would not only lower the price but also still provide high MPGs in the 60+ mpg category. I believe the real reason for the absences of a diesel hybrid is that car makers have concluded that first the EPA hassle with emissions plus the possibility of low sales are to blame. Never understood why all other countries which have signed off on lowering air pollution all allow the sale of diesel cars.
whats this about combined mpg. weather gas or diesel, the internal combustion engine in a hybrid constantly runs at the most efficient rpm. So the author of this article apparently didn't do his homework on what a hybrid is. sheeet if you find vehicle labeled hybrid that doesn't operate like that, well that's what you call false advertising.
Railway (-road) locomotives, in places where it is not economic to run electric overhead cables, running at near constant rpm, have been using diesel electric technology for 50 years, very efficiently. arguments from people who consider the tax inclusive cost of fuel need to be discounted. Taxes distort - sometimes or often deliberately - the real efficiencies. Diesel electric is a mature technology with a bad press. For people who drive long freeway miles, needing the occasional burst of overtaking power, the torque of the (turbo-) diesel plus its efficiency at medium rpm for charging are unbeatable. Note that in Britain, new diesel sales are now greater than "gas" (-oline)
Wow, another person with a brain out there.... I can't believe so many people ever believe the press, ever!!! Thanks for your comments.
Sad that you skirted the key issue in Diesel Electrics. We need to talk about biodiesel electric, not diesel electric. With biodiesel you obtain a near zero carbon footprint. GM is just one step away with their Volt II. Replace the gasoline engine with a biodiesel and GM can off the first zero carbon footprint electric car. True, availability of biodiesel is low at present. But California already had 20 biodiesel stations. A whole new industry springs up, making biodiesel fuel from hemp, etc. This approach is old technology - onboard power generation based on 100 years background in diesel locomotives. The public is being snowed badly by Tesla and other electrics. About 2/3 of them turn out to be coal burners or natural gas burners, operating off the grid. Thermodyamic efficiency is way below modern gasoline burning cars.
I agree. A hybrid biodiesel electric car has many advantages. It surprises me that the possibility of an electric biodiesel car was not discussed in the article. Except for the non-renewable carbon sources used to build the car the operation of the car would be net zero carbon emission and potentially extremely "green". In fact, the best situation would be if biodiesel was used to generate the electrical power while running or off line. Yes, I understand the BTU/lb of biodiesel is lower than non-renewable sourced diesel. To me the big issue is the higher taxes on conventional diesel compared to gasoline. I am not sure how that is justified unless it's to penalize diesel trucks and railroads. If the state and fed. tax on diesel was lowered to what it is on gasoline I would expect the sale of a hybrid electric biodiesel car would dramatically increase. It's about as good as it gets.
I agree totally Bio-Diesel Electric is the future. Diesel engines can run on both bio diesel and conventional diesel if you ad a hybrid to the mix and you are pushing the mileage to unthought-of potentials. Plus, a diesel engine will last 2 to 3 times that a gasoline engine will last. When I was in the Navy we were resurfacing’s some range roads on 29 palms Marine Corp base in California all of our equipment runs on Diesel the base had converted to bio diesel so we just filled the trucks and equipment with the bio diesel and they ran great. The point is there is no special modifications to the engine, you will have to change out the filters. But that is because bio diesel cleans out all the crap from conventional diesel. I think the problem is that Americans don’t know much about diesels they only know the big rigs on the highways.