Why diesels get good economy - application to petrol engines
 

There has been a lot of talk about diesels on this forum, and the fact that they give better economy because they don't have to fight the vacuum of the
engine. Trying the following experiement, on a petrol engine, however, gives some interesting results:

Driving downhill, in 1st gear, at about 15mph, or whatever gives you 75% of
maximum revs, let go of the accelerator, and switch off the engine. You will
get quite a lot of engine braking so make sure no-one is behind you!.

Now, if you floor the accelerator pedal, you will notice that there is no
perceptible reduction in engine braking. However, the engine is now running
like a diesel, at least in terms of no-throttle-plate. I have always thought that, with the throttle plate closed, there will be high vacuum, but the compression stroke will be easy, whereas, with the throttle plate open, there will be low vacuum, but the compression stroke will be more difficult - therefore they cancel each other out!

Therefore, the difference (apart from the higher calorific energy of diesel) must be due to the fact that diesels run leaner than stoichometric.

Has anyone tried running extreme lean-burn, like a diesel?. At high load, or
high RPM, this is likely to cause detonation, and destroy the engine, but at
low revs, it may well be possible. Environmentalists would of course want to add a cat system capable of reducing the NOx emissions this would generate.

This may also be an option with LPG, or hydrogen, etc... :)

i'm quite curios as well

The problem with trying to run a gas engine that lean, even under low load/low revs, is that it would jump and buck so bad it would be pretty much un-driveable. Now, if you could figure out how to totally vaporize gasoline...

I just thought about it - another thing is that diesels have very advanced ignition timing (0 degrees BTDC) due to the way the work... Advancing ignition timing makes more power at the risk of detonation.

I'm going to try leaning the mixture slightly (it runs rich all the time anyway), and advancing the ignition slightly, and see what happens (my car has a det sensor by the way)

Diesels don't have "ignition timing" because there is no ignition system. You get it started using glow plugs and compression takes care of business from there.

Just be careful with your experiment. Lean gasoline mixtures tend to combust faster than a rich mixture, leading to an earlier max cylinder pressure. That's why ignition timing is retarded when your ECU detects knock. If it starts knocking a little you should be OK if you back off, but detonation under high load could do severe damage to your engine. It's not too tough to cross that line and I would hate to see that happen to you.

When I said 'ignition' timing, I mean that the fuel is injected at 0 degrees BTDC (I think), which seems quite good.

I did an experiment - advanced the timing as far as it would go, tuned the AFM as lean as it would go, and heard what det sounds like. There where probably only aboue 15-30 pings in total, and not in a total run!. I have put the timing back to how it was (but a tiny bit more advanced) and have richened the mixture.

I'm now looking at getting a wideband O2 + EGT sensor, which will let me
experiment properly with these settings!.

I did get a good example of the economy increase that may be possible with advanced ignition timing though. The air regulator on my car is stuck in 1 position (I need to fix it), so it idles at 600rpm when cold, and 1200rpm when fully warmed up. With the ignition timing fully advanced, it was idling at 1700rpm. As the air regulator was frozen (which is like the throttle being fixed at the same position), the increased idle speed suggests a significantly increased efficiency.

Anyway, once I get my EGT gauge + Air-fuel ratio meter, and also wire the det sensor to a speaker as noted elsewhere on this site, and get my SuperMID installed properly, I will try some experiments to see the effect of advancing the ignition timing in 1 degree steps.

By the way, my car has 160,000 miles, and the engines are known to be very strong, the only problem being head gaskets (which are still strong but weaker than the rest of the engine). So, I do need to be careful of det - can't afford HG failure / melted pistons / burnt valves etc!

Let us know how it goes. I've attached a chart that's helped me find a balance between A/F ratio and timing. Sorry, it doesn't show actual A/F ratios! It does show the relationship between A/F ratio, EGT, CHT (cylinder head temp), power and fuel comsumption. My best guess would be that the 'best power range' is around 11.0 to 12.99, while 'best economy range' is between 15.0 to 15.99 I'd love to hear any other interpretations of this chart. :confused:

This might be obvious, but knock is much less damaging at low load, to the point of being insignificant. It's also very bad at high loads. It's also not good if you get sustained knock at mid to high load.

I read an artical, I think in popular mecanics about a direct injection gasoline engine that was useing compression ignition once it was warmed up, and useing spark ignition for starting, and the artical was claming lower nox emissions I think, because it would somehow creat a cooler burn, but the other side affect was increased fuel economy.

That almost sounds like an HCCI engine... Did they mention anything about variable CR? I've posted this before, and I might as well post it again, someone did an experiment where they retrofitted a TDI engine for SI/port FI and had better efficiency/lower peak cylinder pressure on methanol and E100 than when the engine was a diesel. So, wrt A/F ratio, each engine probably has some range for best power/economy like DRW said, that's limited by how much heat is left over in the cylinder after each cycle, but with fuels that have higher octane and can absorb more heat w/o igniting, the A/F ratios can be pushed upwards with a nice increase in efficiency.

I found this quote on the SOHC4 forums very interesting....

I have a number of 1.5L CVCC engines, one in my crx hf, one in my '83 civic dx hatch, and a head with bent valves from a friend for poking at, but I never heard of a 750cc cvcc...
1stgencivic.com has a good write up on how the 1.3L and 1.5 cvcc engines used in the 1975-1987 civic's work.

Is not the injection point of the diesel fuel the "timing point" of the diesel engine? The air is squeezed (and HOT!); when the fuel is injected..."Bang!":confused:

Yup, that's how it usually works.

Diesels get good economy for three main reasons. These factors can be mimiced in gas engines:

Diesel combustion is always lean, meaning more complete fuel combustion. Gas engines can lean also out the mixture, but the resulting NOx emissions are excessive, and USEPA would never allow it.

The compression (actually expansion) ratio is higher. Compression ratio in gas engines is limited by knock. A high compression ratio with late-closing intake valve timing that reduce the compression that prevents knock, while still providing a high expansion ratio. This is the "Atkinson" cycle.

There are no part-throttle pumping losses. Variable valve timing can minimize the pressure drop across the throttle.

EGR will abate NOx to acceptable levels. It sure does the trick in diesels.

I have a problem with your question: what does the % vaporization have to do with jumping & bucking? My reason for asking...I can vaporize 100% of the gasoline...but bucking & jumping seem outside the picture of results. Please elaborate....

Vapo-rub
 

I think it's impossible to vaporize 100% of the fuel, with the current technology.

RH77

I could be way off base but also isn't the fact that a diesel optimum rpm for fuel usage also very similar to where the engine typically cruises (2000rpm). I was under the impression gasoline engines tended to turning a lower RPM then what gave you the most mpg from the fuel? I am probably not saying that correctly.

Brock : diesels have peak torque lower down in the RPM range, meaning that you get the most efficient power output while the engine is turning more slowly, so less drag from the engine mechanicals. I think what you are saying is also true, that peak torque occurs at cruise speed, which means you get most efficiency at the speed you tend to drive at.

Yes, that’s what I meant to say. Thanks for saying it correctly ;)

I think you two are making a big assumption. Iirc, peak torque isn't always peak efficiency for gasoline or diesel engines, it usually isn't imle. Diesels just don't have pumping losses at low load, while gassers do. Both have optimal efficiency around 1,500-2,000rpm based on the few BSFC maps I've seen.

Your correct I was assuming that gasoline engines had a higher BSFC then diesel. I have seen them specificaly for the TDI and they are right about 1900-2000, which is where I cruise. I did assume from past knowledge that gas engines were higher like in the 3k range, mabye not so anymore?

I have heard that higher ICE compression ratios are more efficient- perhaps the diesel 22:1 compression ratio helps.

As mentioned in the opening thread, diesels automatically have a respectable 12% advantage. A gallon of diesel has 112% of the btu's in a gallon of gasoline. If we all adjusted our mpg up 12%, that would definitely help to bridge the divide.

1 gallon of gasoline = 124,000 Btu

1 gallon of diesel fuel = 139,000 Btu


Source:
https://www.eia.doe.gov/kids/energyfa...alculator.html

It probably depends on the engine, but imle most are around the 2-3,000rpm range. For instance, minimal BSFC for the TDI from 2-3k rpm is ~200-215g/kwh, but low load (most driving) BSFC is still good in the 230-250g/kwh range. Gassers have the same minimal BSFC (when taking into account the difference in energy density) from ~2-3k rpm of ~230-240g/kwh, but at anything less than full load, the efficiency starts dropping like a rock, and tends to bottom out at something like 500-600g/kwh from 2-3k rpm for most driving.