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.