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Old 05-02-2015, 02:09 AM   #1
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Why aren't hybrids diesel rather than petrol?

Hi, this has been bothering me since i saw my first honda insight in about 2006, and pops up whenever i think about a Prius - why isn't the conventional engine a lovely efficient diesel rather than a regular petrol?!
For maximum mpg, surely harnessing free electric to the already more economical diesel is best?
Weren't submarines always diesel (not petrol) electric, so there can't be some massive problem with using diesel?
Apologies if this subject has already been raised and answered elsewhere, but it is still a huge puzzle to me!
Yes, there's a couple of 4x4 diesel hybrids, but I'm talking normal cars here!
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Old 05-02-2015, 02:11 AM   #2
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I forgot to add my recently fixed badge!
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Old 05-02-2015, 08:38 AM   #3
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I've often wondered this myself, they are usually mated to a rather inefficient petrol engine, almost counterproductive as if one cancels the other out!

The reasons I can think of are: Diesels take up more room, dpf's, turbos and intercoolers etc which also makes them heavy, again not a great combo when you're trying to improve economy.

There quite a few diesel hybrids available, they're just not marketed very well as a regular diesel offers similar economy. Peugeot, CitroŽn, Volvo, Audi, BMW etc all have diesel hybrids. They make far more sense to me, diesel on the highway to reduce fuel consumption and electric in built up areas to slash emissions.
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Old 05-02-2015, 08:43 AM   #4
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By the way, the French hybrids are actually really nice looking cars!
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Old 05-02-2015, 11:22 AM   #5
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Yes, they are - and i've never heard of them! I often feel i betrayed CitroŽn when i bought my Hyundai new, as their second hand old diesels had beaten their manufacturers mpg claims - unlike today!
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Old 05-02-2015, 11:26 AM   #6
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Oh, and its back to unit conversion disaster...i changed our Honda CRV from USgallons to litres, then entered the 30.1 litres i used to travel 380 miles and acheive 44.25 mpg. 57 mpg appeared on my dashboard....i ve had to temporarily stick a fake 39 litres on the fill up to show 44.3 mpg. I ll sort it out later!
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Old 05-04-2015, 07:37 AM   #7
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The tl;dr for why there is few diesel hybrids.
A diesel would add to the final price for what may not be as large of a gain in fuel economy.
Hybrids aren't just fuel efficient, but low emission. Making a diesel one as clean further adds to the cost.
The two biggest markets for hybrids, Japan and the US, favor gasoline.

Locomotives and some ships are serial diesel electric hybrids, but are so for reasons besides efficiency. Directly driving the wheels on a locomotive with a diesel would require a large, complicated transmission. If such a beast could physically fit on the locomotive, it wouldn't have been an improvement over the steam engines still in use up to the '60s. I don't know if a diesel electric was more fuel efficient, but their design simplicity made them for cost efficient over a steam locomotive in terms of crew and maintenance.

For ships, a diesel-electric system meant that the engine didn't have to be in the aft and inline with the propellers. It could be positioned midship for better weight distribution. Electric motors also allowed for steerable propellers in place of a rudder. Some can spin 360. So the ship becomes more agile, comparatively. Subs had batteries so they can dive, and the electric only propulsion is quieter than nuclear.

The conversion losses of serial hybrids make them unattractive for a car. The Accord hybrid is the closest to a serial hybrid available, and it still directly connects the ICE to the wheels on the highway. The i3 REX is a serial hybrid, but is a plugin first. With 70 miles of grid powered range, the efficiency losses of a serial hybrid become less important. In general, the efficiency of a plugin's range extender becomes less important the longer the grid EV range is.

You need to keep in mind that the current hybrids originally were develop not to just reduce fuel use, but also emissions, and not just the carbon one.

In Japan, the government, through MITI, ran several LEV(low emission vehicle) programs to R&D reducing car pollution in cities. These go back to at least the '80s. They experimented with EVs, hybrids, and FCVs. Without the LEV programs, the Prius likely wouldn't have come to market.

The US had the Partnership for the Next Generation of Vehicle. The concept cars produced by the big three were diesel electrics, but they were never required offer anything for sale. So with the diesel engine, hybrid system, and light weight components, production versions would simply have been too expensive. ford did start work on their Escape hybrid during this program, and a government research was perfecting a hydraulic hybrid.

The hybrid premium discussion has dogged hybrids since they arrived. A diesel premium would just make it worse. More so now that diesel costs more in the US. That diesel premium will be higher getting it as clean as the gasoline.

The gasoline engine in most is quiet efficient. The are Atkinsonized for greater fuel efficiency. They lose power, but got enough for steady state cruising, and have the electric motor for low speeds and accelerations. The 4th gen Prius ICE will have a thermal efficiency real close to a diesels. Then the gas ICE and electric motor have better synergy. ICE, high end power; motor, low power. A diesel's power band overlaps more with the motor's; limiting how much advantage came be taken of both in the system. So a lower gain than expected.

Not that a diesel hybrid can't be done, but one of the most successful ones, the Volvo XC60 PHV, to date, is a luxury brand and a plugin. The extra cost of the diesel over a gasoline engine is a smaller fraction of the car's overall price. A non-plugin diesel hybrid should do well in Europe though. As long as the political climate doesn't move against diesels in general.
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Old 05-04-2015, 11:15 AM   #8
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Well I was surprised to see that the UK actually buys more plug ins than any other country in the World. There were actually 52,000 alternative fuel vehicles registered last year, and an incredible 68% people buy cars with carbon emissions of less than 100 grams per KM, about the same a Prius.
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Old 05-06-2015, 01:50 AM   #9
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Diesels can also be very hard to start in cold weather. Diesel engines themselves use much stronger components to handle the higher compression/detonation and thus cost more to manufacture. Diesels in America also have an unjustly earned reputation for being un eco friendly, which is exactly the opposite of the market they are trying to attract with hybrids. Most people who love hybrids equally hate diesels here, mostly because many rednecks here boost up the horsepower on their giant diesel trucks and they billow out lots of soot when they are racing around.


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Old 05-06-2015, 05:55 AM   #10
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The trend in Europe seems to be to skip over hybrids and go right to plugins. I think it has much to do with avoiding taxes and city congestion charges.
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