A lot of people see hybrids and plug in hybrids as a great middle ground between ICE and EV, so why not make a diesel hybrid. They can be used in a variety of vehicles, and can reduce the nox and particulate emissions diesels are known for. So why isn’t there a modern diesel hybrid being produced, at least as a passenger car?
In: Engineering
There are diesel hybrid cars from Peugeot and Mercedes for example. In general diesel engines require more stuff for proper exhaust cleaning and so, comparing to a gas engine, making diesel engines more expensive and heavier.
And in general hybrids are not so green like you would maybe expect as the electrical range is pretty low (normally far less than 100km), no matter whether it has a diesel or gas engine.
For example the Mercedes E300 diesel hybrid has an electrical range of only 45km until you have to recharge the battery. After that you either need to recharge or you use the combustion engine.
there are multiple reasons for this.
first of all diesel engines have a very low RPM range to work with which is something you dont really want.
Beside this Diesel engines have major emissions problems that almost always require the engine and exhaust to be very hot in order for the emissions systems to work and even with that they still need Diesel emission fluid to meet their emissions.
You will also notice that the vast majority of **good** hybrids are not using turbo charged engines as the turbo lag is also something you want to avoid as well as the complexity and reliability problems of turbos which can be entirely avoided by not using them.
Overall the characteristics of a naturally aspirated engine combined with an electric motor are simply the best of both worlds as the electric motor makes up for the low torque and power of the combustion engine at low RPM while the combustion engine itself can output peak power at the upper end where smaller electric motors run out of steam.
And beside all this theres also the fact that diesel engines are more expensive so this would make hybrids even more expensive then they already are.
Gas engines are much simpler to build. Given that you want the gas engine in a hybrid to run as little as possible, the advantages in fuel consumption do not outweigh the added complexity.
Second reason: Diesel engines have good torque at low RPM. You don’t need that in a hybrid.
Third reason: those hybrids are designed as world cars. The US market hates diesel and thinks diesel is only for trucks – heavy trucks, not pickups which are so popular there.
I’m sure it’s possible, but hybrids are specifically paired with a type of engine called an “Atkinson cycle” engine. These are highly efficient on their own – which is why they can achieve 35+ mpg on the highway with gas, when the electric motor is not in use – but have poor “low-end” acceleration. They can’t start from a standstill very well, and that’s when the electric motor takes over.
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Because in order to eliminate those particulate diesel emissions you need a lot of pretty expensive gear and a hybrid engine by its nature is also very expensive.
So most civilian consumers won’t be willing to buy what becomes a very expensive car for marginal savings in fuel economy between diesel and petrol.
The torque VS speed profiles of diesel, petrol and electric engines also show that diesel-electric hybrids are somewhat suboptimal.
Diesel engines excel at sustained medium speeds so work best on long distance endurance travel, while electric engines are best with low speeds and frequent accelerations/deceleration so inner city travel.
The two profiles conflict whereas petrol engines work best at high speeds like that on highways.
However diesel-electric engines do have a place in moving big and heavy things around like train locomotives, some tanks, ships and submarines all of which frequently use diesel-electric engines.
I work at a car company, and getting clean diesel emissions is complicated. Between the sensors, computers, def system, and heat management, small car diesel engines are significantly more complex than standard turbocharged gas engines, which are almost as efficient and way cheaper to make. Combine that with the battery and other electric systems, and a hybrid diesel would be a nightmare to engineer and manufacture.
Diesels have advantages for heavy duty trucks and towing, so we still make plenty of them, but there really isn’t a market for a more expensive hybrid that would likely also break down more often.
When you are marketing EV, gasoline tailpipe emissions look clear. Diesel emissions can look sooty, not always but can. There is a huge branding disconnect there.
Might as well run a coal-fired boiler in there to charge the batteries… suddenly feel the ick factor there? That’s why it’s gasoline.
It might be interesting if they looked at making an alcohol fuel system. If you can’t find fuel stations then pop in the corner convenience store and get a couple bottles of vodka or a case of rubbing alcohol to get you home.
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A lot of comments here points to the emissions regulations rather than efficiency is on-point. Diesel engines have a lot worse cold-start emissions than a gasoline engine would. For a hybrid the engine is effectively cold-starting multiple times per drive, only getting up to operating temperature for the catalytic converter in extended operation..
Diesel-Electric locomotive is standard for almost a century, and it’s a proven technology when paired up. It’s just in an automotive use cycle, diesel engines just don’t operate well in the frequent on-off cycle.
It is only great if the driver is an engineer and understand the drawbacks and benefits of the Powertrain. Also you need a specific use cycle.
Let’s imagine I live 50 km from my work and my diesel-hybrid has an electric range of 45 km. This sucks and I will just torture the diesel engine unless I don’t use the battery and run it like a non hybrid diesel which means I get no hybrid benefit..
If my daily commute is well within range of the battery and I drive carefully so I don’t even start the diesel engine daily I have a great car that can also run long journeys on a weekend with its efficient diesel engine.
I have used a Merc E300de for a 30km commute and I drove it electric 90% of the time. When temperature dropped to -15C it usually started the diesel instant as I hit the start button because the car judged it better get it heated up, now the diesel engine wouldn’t really turn off again even if I had battery power available. But this was probably for the better on those really cold days. Drove the same car Stuttgart to Stockholm with 5.x L/100km don’t remember exactly but it was a good number considering its a large car we never stopped to charge it and we did 140-180 kph all the time in Germany 😄
But it’s really obvious in the Merc 300de anyway that once the engine starts it stays on for longer than in a gas hybrid. It is programmed to maintain engine temperature and also tries to predict the user so it avoids to many start/stops. But the car is fine to drove fully electric if destination is in range imo and the electric engine is strong enough to not test my patience anyway!
If your the kind of hybrid user that will do a kickdown on every major acceleration and start your engine 5 times in your daily commute the Mercedes diesel hybrid is not for you.
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