Yeah but then you are driving a shitty underpowered diesel.
The real question would be :
Why aren’t the euro manufacturers building hybrid diesel powertrains? A small modern Diesel engine coupled to an electric motor would certainly provide more mpgs.
Because Atkinson cycle gas ICE has about the same efficiency as a diesel ICE (about 40%), but is a lot simpler and cheaper to produce. Gasoline cost is just one factor. There are also maintenance costs and turbodiesels lose pretty badly there. It is also harder to get emissions other than CO2 low in a diesel engine (you need DPF, EGR, all of that adds complexity and decreases durability).
I feel pretty well placed to answer this as in my country most cars are diesel, my summer project has been fixing an old diesel engine, and I drive a Prius :-)
- Diesel engines are more complicated than petrol engines. You have a high pressure fuel injection system and turbo(s), which both need significant maintenance approximately every 150,000km. If you don't you'll end up burning excess fuel or oil (that's what happens when you see a diesel vehicle with plumes of black or white smoke under acceleration) which is obviously not good in terms of pollution. If you leave it, eventually your engine will blow up (the turbo will leak oil into the intake air leading to hydrolock, or injectors will put in too much fuel wearing down the cylinder). Compare this to a petrol engine, where maybe you need to replace the spark plugs, and if you don't the worse that could happen is misfires or the engine simply not starting.
- Diesel engines emit a lot more NoX emissions than petrol which needs complicated filters in the exhaust to remove. DPF filters are usually removed after 150,000km because they end up clogged (causing loss of power, higher fuel consumption, and can damage the engine). You can't actually buy new replacements - I don't know why - and 'regeneration' doesn't really work, so they are usually just cut out. In Europe NoX is regulated by the engine class (e.g. Euro 6) when it is manufactured, not during regular emissions checks, so this doesn't cause you to fail that. AdBlue is slightly better, but there are already devices you can plug into cars to disable the system, which many people (and companies) do.
- Diesel engines don't operate well when cold. All modern engines have glow plugs (electric resistance heaters) which heat up the engine block when you turn on the ignition. I imagine this wouldn't work well with a hybrid system where the engine doesn't run as often.
- Diesel engines have a much higher compression than petrol, and use compression to ignite the fuel. This means the cranking power needed to start the engine is a lot higher. For parallel hybrids (like the Prius) this shouldnt be an issue, as the same motors for driving are used to crank the engine, but I imagine it may be more tricky to make that transition smooth compared to petrol. Same when the engine cuts off, as you don't just cut power to the spark plugs to stop it.
- Diesel engines are only usually available in larger sizes because it's not economical to produce a 1.0 litre diesel engine. This would mean a diesel hybrid would be more expensive than a petrol hybrid, and would probably need a larger engine bay.
- Petrol hybrids usually use the Atkinson cycle which is more efficient and provides more torque towards higher RPMs. The electric motor provides the high torque at low RPMs so it works well together. Diesel engines typically provide higher torque at lower RPMs, and need a turbocharger to provide any useful torque at higher RPMs, so it's the opposite of what a parallel hybrid needs. I'm not sure if there are any alternative cycles for diesel engines that would work better for a hybrid.
- After VWs scandal I feel diesel has lost a lot of political clout.
That said, I am jealous of the Audi A2 1.2l 45kW diesel which has a much lower fuel consumption than my Prius :-)
Not in city driving. I own a hybrid Highlander and in city driving it burns considerably less gas than Ford Galaxy or VW Sharan with a diesel engine, despite being bigger, heavier and more powerful at the same time. I agree that on a highway hybrids are very close to diesels in terms of mpg.
