Diesels the best way to get high MPG?
 

Super high MPG cars we may never see - GasSavers.org - Helping You Save at the Pump Hypermiling and Fuel Efficiency Forum

There are a few cool cars in there. A lot of them are diesel/turbodiesel. Is diesel the best way to get high MPG? Is that because of the efficiency of the much higher compression ratio?

Yes, yes, yes! At least for right now. If you look at the new gasoline/direct injected engines, it's all diesel technology using a different fuel and a spark. Mazda's Sky Activ ( gas version ) uses 14 to 1 compression, direct injection and other jiggery pokery to do the deed.

Part of it is the high compression ratio, the other part is the energy density of the fuel. Diesel fuel has more energy per gallon than gasoline. The cam timing usually trends toward making maximum torque at low engine speeds, so you don't have to rev it as high.
So the combination of all these elements yields a high mpg device.

Plus they're not throttled.

The downside is that usually a diesel engine that has the same "right foot power" (not rated horsepower, but equal driveability) will probably be heavier than a gas engine. But I think most people can overlook that.

I really like my new TDI Golf, but your driving conditions will dictate your optimum drivetrain. Unfortunately, it doesn't do anywhere near as well in stop-and-go city traffic as hybrids do. I do mostly highway miles, so it's the right tradeoff for me (especially if I can kill the karman vortex street somehow; I'm working on building up enough 70mph mpg data so I can tell whether airtabs make a difference).

Diesels also have an inherent advantage in the constant pressure expansion of the power stroke in the diesel cycle. You get more work out of the same fuel energy than you do in the Otto cycle all other things the same.

I agree with pretty much everything that's been said. The biggest factor is the cycle/compression ratio. Throttling is big too. Just to add a few things.

Diesels (typically) run at slower rpms as well, so there's less friction. Also, most modern diesels are turbocharged so it allows for the recovery of some of the exhaust heat.

I'm not so sure diesels have less friction. In spite of turning lower rpms, they typically have longer strokes; interestingly enough there are many gassers that run 1000-1200 ft/mn piston speed while the two stroke diesels anyway are running 1700! The ring pack is the largest single contributer to friction so higher piston speeds must lead to higher friction.

Plus they have heavier rotating/reciprocating parts which add friction.

Comparison of Diesel and Petrol engines (yield)
 

According to Wikipedia

Engine efficiency - Wikipedia, the free encyclopedia

thermal efficiency of a modern petrol engines is about 18 to 20%, meaning they convert 20% of total heat of petrol combustion into movement.

On the other hand, diesel engines have a thermal yield of about 40% .

From the best of my knowledge, the main reason isn´t for rpm or friction or even the fuel energy(but they count also), but for the thermodynamics curve of the diesel cycle. It works at higher pressure that needs a fuel that do not detonate at that high pressure.

Turbos are allowing constructors to make smaller and lighter engines with a lot more power, while other advances are allowing them to supress noise and vibration a lot.

It is common to see Jeeps with diesel engines that used to had 2500 cc that turned to 2200 cc producing more HP than their predecessors.

OldBeaver

The diesel motorcycles in the 2011 Vetter Challenge got better fuel economy than the gasoline bikes despite having much bigger engines. The top 3 diesel bikes got 128mpg , 110 and 90 respectively while the best gas bike ( 250 NiNja ) got 94 mpg. Diesel has a lot of advantages as stated above over gasoline engines.

Now that I think about it some more, it might not be fair to make a blanket statement that diesels have lower friction. It probably depends a lot more on the specific engine design. Plus, diesels have higher cylinder pressures (which would increase friction).

i don't think desiel ngine economy is helped by the constant pressure combusion, it would be better if it was at constant volume. Deisel engine consumption is hurt by not having a stoiciometric mixture, let me explian like this: the efficiency of an engine, all else being equal, is related to the ratio of the maximum temp to the minimum. The minimum depends greatly on the expansion ratio, and here the diesel does well, but the highest temperature at the top of the stroke is accieved with a chemically correct mixture, and also in the shortest time.
With a diesel with constant pressure the fuel is injected over a period of time, lowering temperatures and efrficiency over what it could be, and also the fuel that burns further down the power stroke is expanded with a lower ratio.
neverthe less, I have 3 Priuses, and have used these and also a deisel Pajero for my 4km trip to work in traffic, and the diesel actually is not much heavier than the hybrids like this, and it's also lighter than my Daihatsu charade petrol! My favorite engine is the Isuzu 2.5 non turbo direct injection deisel in a ute I had, it got 14km/L in all conditions. Deisel's gains in efficiency more than compensate for any higher losses.....

Volkswagen developed a turbo diesel engine which is not common rail, and seems to have better combustion control. It is the "pumpe düse" (PD) engine:

http://www.myarchive.us/richc/VW_TDI_with_PumpeDuse.pdf

I have a Skoda Octavia 1.9 TDI PD and seems to have a better fuel use than other diesel engines, probably because of the extremely fine control of fuel injection to produce maximum performance.

OldBeaver

If both engines ran the same compression ratio, you would be correct that a diesel is less efficient. This page explains the details better than I can here:
http://www.classle.net/sites/default...ual_cycles.pdf

Yes.

Diesels may be more efficient (thermally), but EM's Garage shows that more important than what you drive is how you drive it. The question remains, can the best gasser hypermilers get the same/better numbers with a diesel (all else being equal), and vice versa?

And yes too.

My latest issue of AEI magazine had an interesting engine from Honda. It had a linkage on the crank that allowed for a larger expansion ratio than it's compression ratio. IIRC, it was a gas engine with 12:1 compression and about 17:1 expansion. Between that and a heat exchanger built into the catalytic converter, they were trying to hit 50% thermal efficiency. I don't remember if they achieved that goal or not. But, it looks like we still have room to improve on gas engines.

I could dig it up tonight if people are interested.

When the higher energy content of diesel is taken into account, the numbers aren't that good anymore.
Then there's the matter of the worse emissions: NOx, particulates.

The sheer complexity of today's diesels is another problem area.
I'm not the only one with particulate filter issues.
Current research indicates that having a DPF may well be worse than having none - the particules are still there, but smaller and more harmful as a result.

I don't know of any one with a big gas powered truck getting MPG numbers in the high 20s to low 30s.

It hasn't been mentioned yet, but diesels can also be made to make obscene power levels and are virtually indestructible (at least in trucks). My 5,000 pound, shaped like a brick diesel truck just made 854 rear-wheel horsepower, and 1,754 lb-ft of torque on a mostly stock (head studs and o-ringed head) 230,000 mile engine. On the drive back with 3.07 gears and an OD at 65mph, it got 27mpg.

True, no 800+ horse power gas truck is going to get over 12mpg.
Over 1,700ft.lb. of torque is never going to happen on a street legal gas powered vehicle.

Yes this is true for pickup truck diesel engines, but not so much for VWs. A 1.8l VW gasser engine can support 400hp with more fuel and boost, but the VW 1.9 TDI engine can only support about 200hp reliably (without internal work). And you would be lucky to get 200hp out of an old 3.0l Mercedes 617.952, but some of the crazy Finns have done that.

Maybe not a production vehicle but.... Jay Leno's - 1934 Rolls Royce Phantom II powerd by Merlin engine. | Flickr - Photo Sharing!

Nice! I rode in a (P-51) Mustang powered by that engine. 1490 HP @ 3000 RPM. What a ride!
Well, this Mustang wasn't street legal, but it had FAA approval for flight. ;)