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Diesel Hybrids: Why They Don't Make As Much Sense As You Think

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Mercedes-Benz E300 Bluetec Hybrid sedan

Mercedes-Benz E300 Bluetec Hybrid sedan

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Volkswagen's line of TDI diesel cars has passionate fans.

Now that the company is launching the 2013 VW Jetta Hybrid, it will likely gain some hybrid advocates too.

But diesel and hybrid fans are very different groups, as the company's marketing surveys show.

So why couldn't Volkswagen simply add a hybrid system to its diesel, thereby getting the best of both worlds?

Many diesel fans have suggested just that, but there turn out to be valid reasons that automakers believe will largely keep diesels and hybrids separate.

In discussions with engineers from Volkswagen and other diesel makers, three main hurdles come to the fore:

(1) COST

First and foremost is the issue of cost. On average, a diesel engine costs about 15 percent more to manufacture than a gasoline engine of equal output.

Add to that the $1,000 or more for a high-voltage battery pack, power electronics, and one or two electric motor-generators, and you've created a very pricey powertrain indeed.

In European countries, where diesel fuel and gasoline can cost $7 to $10 a gallon, buyers will pay a considerable premium for more fuel-efficient cars.

That's less true in the U.S., where gasoline averages less than $4 a gallon--and where diesel fuel is often more expensive per gallon than gas.

And it's one of the reasons that diesel passenger cars pose a thornier equation in the States.

While their torquey driving characteristics make diesels appealing (just like electrics), the fact that both the cars and the fuel are more expensive makes the payback from diesel's greater fuel efficiency more challenging to compute.

(2) NON-COMPLEMENTARY TORQUE CURVES

Most hybrids, especially those from market leaders Toyota and Ford, use gasoline engines specially tuned to run on what's called the Atkinson Cycle.

This highly efficient tuning gives them maximum power output at the top of their range, but almost no torque at lower speeds.

That is perfectly complemented by a hybrid's electric traction motor, which develops peak torque at 0 rpm, neatly compensating for the gutless gas engine until it runs up to speed.

A diesel, on the other hand, produces all its torque down low--as does an electric motor.

That means a diesel hybrid should have boatloads of torque off the line, but may require extensive gearing to ensure highly efficient running at speed.

There's likely a more technical explanation of the relative power and torque curves of the three different sources, but we're not going to attempt it here.

2013 Volkswagen Jetta Hybrid

2013 Volkswagen Jetta Hybrid

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(3) LESS IMPRESSIVE IMPROVEMENT

Finally, one of the reasons that hybridizing gasoline engines works well is that they're less fuel efficient to start with.

Gasoline engines convert 25 to 30 percent of a fuel's energy content into forward motion at the wheels; the rest is wasted as heat and noise.

By contrast, a diesel converts 30 to 35 percent of the fuel's energy into forward motion--hence the higher fuel efficiency figures.

But that leaves less "headroom" for improvement.

In the end, say diesel engineers, a diesel hybrid would add a very expensive electrified system to an already-expensive engine ... but produce a less impressive overall increase than in gasoline cars.

Aha, but you say, there are already three diesel hybrids on the market in Europe!

Well, that's true. But only one of them pairs the diesel engine and electric motor together into a single powertrain: the Mercedes-Benz E 300 BlueTEC Hybrid.

The other two--the Peugeot 3008 HYbrid4 and the Citroën DS5 Hybrid--are what is known as "through-the-road hybrids," meaning that they have one powertrain on either end.

Those two can use the electric motor alone for lower-speed trips, the diesel engine for high-speed travel, and combine for highest performance driving by transmitting power through all four wheels.

It's probably significant that Mercedes-Benz, which has sold diesels in the U.S. for many decades, has no plans to sell the world's sole diesel-electric hybrid powertrain here in the States.

We'll see how it does in Europe once it's been on sale for a year or two.

Meanwhile, we invite engine and powertrain engineers to weigh in on these issues.

Leave us your thoughts in the Comments below.

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Comments (61)
  1. This is accurate. When you drive 60%-99% on electricity (as with most Volt owners), who cares if the other part is diesel or gasoline? It's largely irrelevant.
     
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  2. I care, and very much so: diesel engines last two to three times longer than gasoline ones, because they are inherently simpler: less things which can break or go wrong, especially if you are like me and keep running your car, well, forever.
     
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  3. Well, my co-worker's Camry four cyclinder gas engine with over 320,000 miles are still running strong...
     
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  4. There are Mercs that you count the mileage by the times that they've maxed the odometer :P
     
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  5. *diesel Mercs
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  6. Sure, but changing a water pump or alternator on a Mercs with diesel will cost more than a complete engine rebuild on a Camry...
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  7. Very few cars die from direct engine wear that isn't caused by neglected maintenance. Most are scrapped because the cost of failed components exceeds the cost of finding a functional replacement.

    And high-end cars are more prone to that sort of death as repair costs for major components (e.g. A/C, suspension, ABS components) are often quite breath-taking, even when the engine and transmission are quite functional when the vehicle is scrapped.
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  8. I had several TOYOTA vehicles once upon a time, the last one had 236,000 miles when I gave it away and was still running although the rings needed changed.

    But that is nothing for a diesel engine. My mazda6 started breaking in at around 70,000 miles, and I expect to get at least half a million miles from it with the rigorous maintenance schedule.

    Diesels don't even start to break in until 60,000 - 80,000 miles, and it is expected that the odometer will roll over several times during the lifetime of the engine.
     
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  9. Annatar - While that used to be true on older diesels with mechanical injection, and before the widespread use of turbochargers, modern diesels don't quite have the same reputation yet. They're partly hampered by the added complexity of high-tech electronics, urea injection systems, particulate filters, and often-necessary features like dual-mass flywheels to damp vibration.

    The engines themselves are no doubt still reliable, thanks to ever improving production techniques, but they're often let down by short-lived ancillaries. It's little use having an engine that lasts half a million miles if you've had to pay for several expensive replacement parts in that time.
     
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  10. "While that used to be true on older diesels with mechanical injection, and before the widespread use of turbochargers, modern diesels don't quite have the same reputation yet. They're partly hampered by the added complexity of high-tech electronics, urea injection systems, particulate filters, and often-necessary features like dual-mass flywheels to damp vibration."

    Maybe not here, but in Europe for example, mazda has had the mazda6 with a particulate filter available since 2005, and those engines are near-indistructible.

    I know what I have and I know their quality; I do not need it proven to me, because I used to drive those cars every day as my daily driver, for years.

    I have lived in the future.
     
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  11. Annatar - Again, as I've explained before, I live in Europe myself, and am well aware of the pros and cons of modern diesels.

    Your Mazda6 may well have been impeccably reliable, but the briefest of looks at many an owners' forum is enough to see that incidents are hardly isolated - modern diesels are very complex indeed and expensive to fix when they go wrong.
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  12. If diesels are "inherently simpler", why do they cost more to manufacture than a gasoline engine?
     
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  13. They do not, that is a marketing lie.
     
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  14. The components that stand up to the higher compression and higher cylinder pressures/temperatures are more expensive and require pricier materials. When a manufacturer ignores this reality, the outcome is a reliability and durability disaster such as the 80's Oldsmobile Diesel.
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  15. With about twice the BMEP, the whole bottom end has to be noticeably sturdier... usually forged crank and rods, plus bigger bearings and a stouter block. That's some $$ right there. Then, the fuel system has to accurately meter quarter-drop shots of fuel at 20,000+ psi, meaning micron-tolerance machining of high-strength alloy parts. More $$. Next, to get enough VE for modern power demands, add one, often two turbos... plus associated high pressure ducting, intercooler, wastegate or VNT, and actuators. Ka-ching! And finally, diesel smog control isn't just an elegant 3-way cat... add to that the particulate trap and urea injection system. It's an impressive engine, but it's a long way from the 240D, too (thank heavens).
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  16. There are more comments in this thread
  17. "Gasoline engines convert 25 to 30 percent of a fuel's energy content into forward motion at the wheels; "
    "By contrast, a diesel converts 30 to 35 percent of the fuel's energy into forward motion--hence the higher fuel efficiency figures."

    Hmm... I am NOT sure that is "entirely" true. It is true that diesel has a higher compression thus producing energy at a slight higher efficiency point. But Diesel fuel also has more energy content than gasoline to start with on a per gallon basis...


    I completely agree with Point #1 as the most important reason. Point #2 is also valid since #1 is dominating. If they aren't paired well, then why bother.

    I think diesel is better paired in terms of "series" hybrid.
     
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  18. Also, the frequent start/stop city cycles for hybrid isn't ideal for diesel either. (unless you do a series configuration).

    I think clean diesel might be a "good engine" for Volt's series configuration (with special cases for parallel application at higher speed in extended range mode). But that will make Volt even more expensive.

    Since Cruze is coming with a diesel engine. If that does well, I don't see why GM can't put it in the next generation Volt.

    However, with the diesel engine, it would be difficult to meet California's AT-PZEV emission standard.
     
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  19. A diesel Jetta uses about 200 fewer BTUs/mile as a gasoline Jetta. So yeah the diesel is more efficient.

    The Cruze and Volt are both ULEV rated when running on their gasoline engines. The Jetta TDI is also ULEV rated. In other words equally clean whether gas or diesel.

    Ford/Honda both claim they can make a SULEV or PZEV diesel easily; especially since the sulphur was removed from the fuel. And the start/stop is no big deal. The Lupo TDI had the technology back in 2002.
     
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  20. Good all-around article explaining the biggest issues.

    One of the main show stoppers for me is the fact that so far, with the exception of 2014 mazda6, diesel hybrids have exclusively been automatics. And as I have written before, I would rather walk or ride a bicycle than drive an automatic ever again.

    If there were a diesel hybrid sportwagon with a manual transmission, it would be a viable alternative, at least to me.
     
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  21. Why does it have to be manual in a hybrid? I drove my manual for over 20 years. I loved it. But with my Volt's direct drive system, I don't miss it at all. Instant torque anytime...

    With manual, you get to control the shifting point so the engine is always "singing" at the best rpm with most of the torque. With direct electric drives such as Volt, you get that max torque response at any rpm and any speed...
     
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  22. "Why does it have to be manual in a hybrid?"

    It has to be a manual in *everything*, not just in a hybrid. I will not pay for a vehicle with an automatic transmission - I suppose you can call it good old fashioned American "voting with my wallet", if I do not have any other say in the matter.
     
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  23. Shifting is a characteristic of ICE, not needed at all for well designed electric motors.

    You might want to do it for fun. But it has nothing to do with the technology.

    This is NO different from people who still "love" to ride horses, just b/c we have automobiles now...
     
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  24. "Shifting is a characteristic of ICE, not needed at all for well designed electric motors."

    I know. And I loathe the very thought of it. Driving will become horrible.
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  25. I disagree...

    Try a Tesla S if you can...
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  26. I've always driven stick-shift, always hated automatics (esp their slimy torque converter), so just like you, I was dead convinced that any vehicle without a clutch pedal and a stick would hopelessly suck.

    Until I test-drove an EV.

    Mind blown. It's so unbelievably responsive. Your right foot is like directly connected to the road. You get maximum torque/power all the time. Accelerations are uninterrupted. It's so fun to drive.

    Now that was for a pure electric drivetrain, where the motor is permanently connected to the wheels (no clutch anywhere).
    Serial hybrids should be just as good. Parallel hybrids have a lot more stuff in the way (planetary gears & co), and often smaller electrical motors, so your concerns would remain for those.
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  27. Why do all hybrids have to be automatic? Honda makes the CRZ with a manual shift, and they used to make the Civic and Insight with manual shift too. That gives them better fuel economy (no torque converter/slushbox) and easiert maintenance/repair when something goes wrong.

    As for the article: Well that's nice opinion, but someone has BUILT a diesel hybrid car. They replaced their Honda Insight's gasser engine with a 1.2L diesel engine. It now gets over 90mpg on the highway (100+ with a skilled driver) and has great acceleration thanks to the electric motor assist.

    Who here wouldn't want a 90mpg diesel hybrid car?
     
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  28. The misconception that manual gives better fuel economy is a moot point now. With advances in tech and 7-9 gear transmissions/CVTs, autos actually out do manuals in mpg in most cases. People chose manuals these days only on the basis of cost and the fun of driving a manual.
     
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  29. @Annatar: Thanks for the good words.

    I think when you say "diesel hybrids have exclusively been automatic" you mean (non-hybrid) diesel cars? There aren't any diesel hybrids sold in the U.S. today, and I'm not aware that there will be for some time.

    The 2014 Mazda6 is not a diesel *hybrid*, nor is the 2014 Mazda CX-5 that's expected to be the first Mazda to use the SkyActiv-D diesel engine.
     
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  30. No, I mean the current diesel hybrids offered worldwide - the Volvo, the Peugeot, the Mercedes. None of those diesel hybrids offer a manual transmission as an option, and that rules it out for me.

    When I talk about cars, I am almost never looking at just the North American market, and almost always looking at what is available elsewhere in the world, because my goal first and foremost is to have those same choices here in America.
     
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  31. Then what is the i-ELOOP system used for?
     
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  32. Its to reduce the load on the engine from having to power electrical accessory items in the car like the stereo or lights.
    The system charges capacitors during deceleration then uses that energy for the electrical system reducing the load on the engine for not having to run the alternator constantly from the engine.
     
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  33. @John, good summary. I'm not a powertrain engineer, but the points in your article are consistent with my understanding. A diesel hybrid is cost on top of cost with marginal benefit.

    The cost of diesel fuel is a big factor. For instance, the EPA estimates annual fuel cost for the Jetti TDI is virtually the same as an equivalent gasoline Focus. This situation is different in some countries in Europe where the cost of diesel is artifically supressed through taxation.

    What we might see at some point is a move away from Atkins cycle to smaller GTDI (like the Jetta hybrid), or even Miller cyle with supercharger.

    I agree with Xiaolong that diesel could be considered for a series hybrid (but a small GTDI might be cheaper/better).
     
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  34. I disagree with the article. Well that's nice opinion, but someone has BUILT a diesel hybrid car. They replaced their Honda Insight's gasser engine with a 1.2L diesel engine. It now gets over 90mpg on the highway (100+ with a skilled driver) and has great acceleration thanks to the electric motor assist.

    Who here wouldn't want a 90mpg diesel hybrid car?
     
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  35. And what about a diesel range-extender hybrid?
     
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  36. Sure, that would be the "series hybrid". But it will add even more cost to it.

    Also, in the US, diesel cost anywhere from 10%-15% more...(except when CA gas was $4.60/gallon and diesel was actually cheaper).
     
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  37. Enlightening, John.
     
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  38. Is any company working on a propane/electric hybrid? I know Honda has a natural gas Civic, but why not add electric motors
     
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  39. There would be several issues with a natural gas hybrid:
    1. Package -- the tank takes up over half of the trunk in the Civic; batteries also are bulky; trying to fit everything in could be difficult.
    2. Cost -- Like a diesel hybrid, you would be adding cost on top of cost and probably price it out of consideration for most buyers.
    3. Infrastructure -- Still an issue on NG vehicles
    4. Range -- Hybrid might help, but range is relatively short in Civic; tank holds the equivalent of about 8 gallons of gasoline.
     
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  40. A propane electric hybrid would be great wouldn't it? No stale fuel, cheapest? Per gallon / litre, can benefit form liquid injection technology (like that used on the ford ecolpi falcon in Australia) and in volt-like setup, can either charge battery purely, or assist if needed (personally I think that the option to power the wheels from an ice should be an added option on the volt, and not come standard)

    And unlike the LNG tanks, LPG / propane tanks can now be extruded to effectively "flat " shapes rather than round cylinders, like has been done on the Holden (gm) commodore (again!) in Australia
     
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  41. Hyundai-Kia has produced the 2009-10 Elantra LPi Hybrid and Forte LPi Hybrid, but only for the South Korean market, where LPG fuel is readily available. I don't know how the packaging issues have been solved, but I suspect part of the solution is that these are not dual-fuel gasoline/LPG vehicles, but LPG only. This would free up the space under the rear seat normally used for the fuel tank for either the LPG tank or the batteries.
     
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  42. A diesel would probably work well in a Volt, but you have to wonder if even a 25% increase in mileage makes the investment worthwhile.

    There are 2 other issues that come to mind;
    1. Keeping the fuel lines warm in winter. Diesel will gel in cold weather so you would have to heat the fuel line quite a bit anytime there is even a chance of needing the engine.
    2. Fuel storage time. The Volt already burns through a tank per year whether you need it or not. I don't know if you would have to burn the diesel off more frequently. My guess is it is more stable, given that it is less volatile, but I am not a chemist.
     
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  43. Mike - On point 2, I think your guess is correct. To my knowledge, diesel doesn't go 'stale' as quickly as gasoline, so you're able to store it for longer.
     
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  44. "I don't know if you would have to burn the diesel off more frequently. My guess is it is more stable, given that it is less volatile, but I am not a chemist."

    You can, and it is. I had my diesel car sit for three years in storage with 75% of the tank, and when I cranked it, it fired right up.

    The only concern is algae formation, but if one is careful to always pump at the same stations where the fuel storage tanks and the fuel is clean, even that is not an issue.
     
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  45. "Sure, but changing a water pump or alternator on a Mercs with diesel will cost more than a complete engine rebuild on a Camry..."

    Changing the timing belt and the water pump is part of the standard maintenance procedure as documented by mazda, at every 60,000 miles (100,000 km), and it cost me the equivalent of $300 USD.

    Rebuilding the engine costs far more, I know since I have done it myself multiple times, on several TOYOTA's no less.
     
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  46. "Your Mazda6 may well have been impeccably reliable, but the briefest of looks at many an owners' forum is enough to see that incidents are hardly isolated - modern diesels are very complex indeed and expensive to fix when they go wrong."

    I think that the above is called "FUD" - "Fear, Uncertainty, Doubt" in computer circles.

    Any car can break. Why are diesel cars with particulate filters suddenly so different from gasoline ones with a catalytic converter?

    I mean, really, seriously? Or is the whole goal of this website is to push electric and hybrid cars at all costs?

    Just for the record, I stumbled upon this website looking for more inside news on diesel cars, not because I care about electrified cars and hybrids.
     
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  47. "Until I test-drove an EV.

    Mind blown. It's so unbelievably responsive. Your right foot is like directly connected to the road. You get maximum torque/power all the time. Accelerations are uninterrupted. It's so fun to drive."

    I know they smoke tire, I know there is instant power on demand, but I cannot get past the fact that there is only the brake and throttle" pedal and nothing else. There is no clutch. No shifting. I cannot get past that.

    And then I think about my baby - my clean diesel mazda6 parked in storage over in Europe, think about how I cannot import it here in the United States, and I feel the foam staring to form around my mouth...
     
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  48. That should have been "starting" not "staring" -- would it be possible to add the "Edit" button?
     
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  49. "There is no clutch. No shifting. I cannot get past that."

    Well, that is your problem. NOT an issue with the technology.

    Riding horses are fun too, but I don't want them on my hwy either....
     
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  50. I am not just going to be pressing gas or brake and turning the wheel left or right; I want to DRIVE. Driving without a manual transmission and the clutch pedal is not driving, it is vegetating behind the steering wheel.

    Out of the question!
     
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  51. I have not read all of the "comments" to this article so appologies in advance if I rehash a previously discussed topic. Why are there no deisel/electrics that use the same principle as locomotives? There could be a small deisel motor turning a gernerator to produce elecetricity to be stored and power electric drive for the car. It seems that would be the most economical hybrid system.
     
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  52. Despite Mr. Voelker's reasoning, the leading automotive authority on the planet (MB) is proceeding with just such a car. Yes, they are expensive but most worthwhile things are. One valid concern is maintaining engine temperature when the diesel isn't operating (diesels operate far less efficiently then gas motors when cold). This could be overcome by circulating engine coolant through the battery bank. Another argument is that hybrids are a short-term solution that will be rendered obsolete by evolving electrical storage and application technology, so why bother. That being said, the MB diesel-hybrid cars will be on the road for 50 years/500,000 miles, long after the Atkinson cycle Prius vehicles have been recycled into iPads.
     
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  53. Mark,

    Thanks for the info. It would seem that the billions(?) of miles covered by diesel/electric locomtives is solid evidence that such a system is an effective solution.
     
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  54. article:
    Well that's nice opinion, but someone has BUILT a diesel hybrid car. They replaced their Honda Insight's gasser engine with a 1.2L diesel engine. It now gets over 90mpg on the highway (100+ with a skilled driver) and has great acceleration thanks to the electric motor assist.

    Who here wouldn't want a 90mpg diesel hybrid car?
    .
     
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  55. the purpose of putting a battery into a fossil fuel powered car is to reduce fuel consumption, not just for additional power. the battery would come ESPECIALLY beneficial in traffic jams that gives you massive headaches. in long traffic light stops, maybe you can turn off the engines to stop burning fuel, thus, you save. but in traffic jams, the vehicles moving in front of you are unpredictable. sometimes you have wait for a long time, sometimes a very short time before you start moving again. its risky to turn off your engine in a traffic jam. so here comes the battery for our savior! with the battery, you are not burning ANY fuel at all but you can still move your car. even if your car is a diesel, when you are not moving, fuel is wasted.
     
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  56. I understand that 2015 audi and MB diesels will all have start-stop added...
    ...so engines will not be running when stopped.
     
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  57. You are absolutely correct in your analysis, but you left one option out, the full diesel-electric drive where there are no mechanical linkages from the diesel engine to the drive train. Like the current D-E locomotives, properly geared, D-E it is the most efficient of the three drives.
     
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  58. Yes!
     
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  59. have anyone check out volvo V60 it's all over Europe goes 150mpg+ tell me that make any Sense!!!! and it's a TDI, that mean you can use your own old veg oil to run it after filter!
     
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  60. The diesel mercs that last 500,000 with no engine repairs are a thing of the past. Mercedes seems to have abandoned their "built to last" mentality for the higher profits that result from the planned obsolesence. Basically any diesel Merc from 1979 to 1995 is going to be bullet proof except the 350SD. The smaller 190 will get about 40% better mpg than the larger models and the turbo charged models are much more drivable under modern conditions. The larger E class and S class get about 24 MPG town and around 30 highway.
    Mercedes does not offer the basic models with manual transmission in the US.
     
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  61. A few things that need to be looked at diesel is no more expensive to manufacture than gasoline, I can remenber when diesel was cheaper than gas. What changed the our governments tax it.
    The second thing is the most efficent diesel hybrid system is currently used on trains by General Motors locomotives.
    Diesel motors are not more expensive to manufacture than gas, they just charge more for them.
     
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