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Old 12-22-2006, 12:03 PM   #1
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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...
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Old 12-23-2006, 12:21 AM   #2
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i'm quite curios as well
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Old 12-23-2006, 05:25 AM   #3
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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...
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Old 12-23-2006, 06:59 AM   #4
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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)
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Old 12-23-2006, 08:29 AM   #5
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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.
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Old 12-23-2006, 12:30 PM   #6
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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!
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Old 12-24-2006, 09:07 PM   #7
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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.

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.
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Old 12-24-2006, 11:14 PM   #8
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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.
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Old 12-25-2006, 04:28 AM   #9
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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....
Quote:
Originally Posted by Hondaman
The 750 engine was in development at the same time as the 1300cc CVCC 2-chamber engine, and some of its swirly design came, I believe, from that relationship. The CVCC was the ultimate forced-swirl design: a tiny little "precombustion" chamber with a tiny intake valve sat on top of the regular chamber. A small passage connected the two, and the 2-bbl carb had one side set to 12:1 A/F ratio, the other side at 17:1 A/F ratio. Two distributors fired 8 plugs on the 12-valve 4-cylinder like this: the pre-combustion chamber fired first (45 degrees early ! ), driving a rich whirlwind down into the main chamber (at an angle) to swirl that upcoming charge really fast. Then, the second spark (more normal advance rates) would fire the richer mixture at the outer edges of this little storm, which would then burn the leaner part for a longer push time on the piston. It was a peaky engine, so much so that many thought it had a radical cam, but that timing was close to our 750s. The result was an engine that would turn almost 9000 RPM in a car, getting 50 MPG at 80 MPH speeds, and would fit 2 Japanese or 2/3 of an American driver. It also made less than half of the emissions of a standard engine, but with no smog controls of any kind! We used to call them the "roller skate" cars, they were so small, but quick.
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Old 12-25-2006, 09:44 PM   #10
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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.
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