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Old 05-11-2007, 08:22 AM   #1
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The reason diesels are efficent at idle? Solved!

I've been thinking about this for a while, and I've done experiements to see the effect of engine braking in my car, which show that full-throttle (with engine-off) seems to make no difference (although it should, as the 'pumping losses') are reduced.

My observations:

(1)

'Pumping losses' seem to make little difference, when I roll down a hill at 15mph, in 1st gear (very strong engine braking), with the engine off. Pushing my foot on the accelerator, or leaving the throttle plate fully closed, makes no difference in the degree of engine braking present. I think this is because:

If the throttle plate is closed (high vacuum), then the engine easily compresses the vacuum in the cylinder, but then has to work to 'expand' the vacuum in the cylinder again. This is an air spring, with the gas in the atmosphere being the 'spring'

If the throttle plate is open (full throttle), then the engine works hard to compress the gas in the cylinder, but then easily 'expands' the air in the cylinder again. This is an air spring' with the gas in the cylinder being the 'spring'.

Therefore, there is almost no difference between the throttle plate being open or closed.

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(2) If the mixture goes too lean (maybe above 17.0) in a petrol/gas engine, then the fuel economy drops (from a graph I was shown on this website in the past). Diesels do run lean at idle, but this isn't necessarily the reason that they are economical at idle (if it wasn't running lean, it would be revving to the redline in an instant - diesels even run lean at full throttle anyway).

----------------------

Therefore, I think that gas(petrol) engines are inefficient at idle, because, when they are running at a high vacuum, only a tiny amount of air actually gets into the cylinder!. Once the mixture has ignited, the final volume is not much bigger than the volume of the cylinder, so, a lot of the energy is wasted (because the ignited mixture will exert very little pressure on the piston, as the volume in the cylinder approaches the volume that the combusted mixture 'wants' to be at.

Diesels, on the other hand, run at full throttle. This means that any extra expansion caused by the combustion of tiny amounts of diesel gets turned into extra pressure, and as the cylinder is at full throttle anyway, a lot more of the extra pressure is 'useable'.

Therefore, when you run a gas/petrol engine at high vacuum (low power output) a lot of the energy is wasted, whereas, when you run a diesel engine at low power output, it still remains efficient!.

When you run a petrol engine at a more significant throttle level, there is more air in the cylinder, so it is running more like a diesel engine, and more of the available energy is useable.

Any comments on this idea?
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Old 05-11-2007, 08:47 AM   #2
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That's more or less what's going on. Low load pumping losses only influence a gasoline engine when combustion occurs, all of the engine braking you experience when the engine is off is actually friction losses, which you illustrated by opening and closing the throttle w/ no change in speed. Pumping losses only happen during combustion. And, like you said, they happen because w/ a gasoline car, there isn't enough air sucked in to have the pressure in the cylinder and pressure in the crankcase equal when the cylinder is at the bottom of it's stroke. So, since the pressure in the crankcase is greater than the pressure in the cylinder, the engine must expend some of the energy it just made overcoming the force associated w/ the greater crankcase pressure. Diesels have no throttles, except for emissions purposes, so they're always sucking in as much air as possible, and they don't have that cylinder/crank case pressure difference. When I first drove a diesel, it was weird because they decelerate like an automatic gasser due to no low end pumping losses, so I ended up using my brakes more than I wanted to, until I adjusted to the longer decel distances.
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I think if i could get that type of FE i would have no problem driving a dildo shaped car.
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Old 05-11-2007, 10:51 AM   #3
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Innnteresting. I agree.

Is there an application to gasoline engines? If we fed nitrogen into the intake to dilute the oxygen, then we could have a larger throttle plate opening at idle... Forget nitrogen, use exhaust gas... Hmmmmm....
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Old 05-12-2007, 11:52 AM   #4
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i also can see that
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Old 05-12-2007, 02:27 PM   #5
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Quote:
Originally Posted by Bill in Houston View Post
Innnteresting. I agree.

Is there an application to gasoline engines? If we fed nitrogen into the intake to dilute the oxygen, then we could have a larger throttle plate opening at idle... Forget nitrogen, use exhaust gas... Hmmmmm....
Exactly. But, the problem with EGR is it's only effective at a higher load, since too much EGR at too low a load can result in pinging due to the extra heat and erratic combustion due to the ratio of exhaust to air/fuel (aka intake charge) and problems with uniform distribution of the charge. One way of dealing with this is to use a fuel with a higher octane, egr cooler/s, and a high compression ratio so the distribution of the intake charge isn't as much of a problem. With this in mind, a gubberment agency took a VW TDI motor and retrofitted it for use w/ gasoline. It had all the properties I just mentioned and managed to have greater the diesel efficiency on both ethanol and methanol, but it didn't run on gasoline because the octane wasn't high enough. Now that there's word of that lean burn catalytic converter, we may not see something like this, but it's definitely doable.
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Old 05-12-2007, 04:12 PM   #6
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If you could pump exhaust gases, from just before the rear silencer, run them through an intercooler (might not be needed), and then into the intake, then it might work!

I suppose we already know the important message - don't idle gas/petrol engines!.

If my idea at the top is correct, then theoretically, 'pulse and glide' while staying in 5th gear, might be more efficient that travelling in 5th at a constant speed.

For what it is worth, my car is an old 8-valve design, and has very little power at high vacuum. More modern engines might be more efficient (due to setting up the fuel/air mixture in a different way? or having a higher compression ratio?). I have driven a 1.3 litre Nissan Micra (K11), which has a 16v engine, and even with slight application of the throttle, at 25mph, in 5th gear, it begins to pick up speed - this car might be more efficient at idle than my Bluebird.
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Old 05-13-2007, 08:56 AM   #7
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I'm guessing Bluey already has EGR, but maybe you could see if increasing the port size helps out noticeably w/ mileage, or try running it through a small IC like you mentioned? You'll need to run stand alone fuel injection with provisions for EGR or have a reprogrammer for your ECU controls, DRW messed around with the EGR maps for his turbo DSM so you could PM 'em about it. Idling/low load is by far the most inefficient engine operation, and P&G where you can accelerate w/ the most load is probably the most efficient method.

The difference in low rpm power you're describing is probably because of the 8v versus 16v/sohc versus dohc differences between the Micra and Bluey. By having two cams, a manufacturer doesn't need to chose between low end airflow/power compared to high end over the same rpm range. For instance, my Camry's engine is a dohc design, and makes peak torque from ~2000-6000rpm, but my sohc truck motor makes peak torque only from ~2800-4800rpm, or half the range the dohc engine makes peak torque over. If Bluey's sohc and had good low end grunt, it would have to give up high end, and vice versa.
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Old 05-15-2007, 07:41 PM   #8
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I stuck a garden hose in my Saturn's tailpipe and carefully metered some of this cool exhaust into the air cleaner at various fixed throttle positions. I could never get a decrease in fuel consumption or an in increase in RPM with the throttle blocked at a specific opening. When you add a little more inert matter to the mixture the fuel charge is so diluted the engine will not fire. I think the Saturn is already getting all the EGR it can handle.

When the diesel engine is idling the fuel is concentrated in a small volume of the combustion chamber when it ignites.
While the overall fuel ratio is very lean it is not very lean at the point of ignition.

The new direct injection gasoline engines do tend toward stratified charge with some increase in mileage.

For us gasoline engine users I think the Atkinson cycle is about as good as we can do. I have tended to favour higher compression ratios with longer duration on the intake valve. I did this on my 1972 Javlein. That was another oil crisis. Crower offered a cam for this during the first fuel crisis.

Do they make high compression pistons for 1.9 Saturns?

When I was in college I analyzed some various combinations of compression ratios and intake duration. 18 months before I saw a press release on the "miller cycle" engine in the Mazda Millennia I wrote to Ford suggesting they investigate such strategies. They responded that they had no interest at all in such a concept. Who knows? I guess it was just a coincidence .
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Old 05-16-2007, 12:01 AM   #9
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The main reason for the increased fuel efficiency at idle in a D is that the A/F ratio gets mega-lean. That's pretty much it. Diesels have wild swings in the air-fuel ratio, whereas with gasoline engines it is a fairly narrow range of AF that they operate in.

In traffic jams, the engine temperature of my smart cdi actually drops dramatically, which is an affirmation of the extremely lean mixture.
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Old 10-12-2007, 07:24 AM   #10
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Diesels don't have a throttle plate and don't really create vacuum. They run unimpeded with air...no limiting throttle plate ect. They inject diesel on a need basis controlled by your foot. That is why they can run at 5:1 to 80:1 fuel/air ratio...that is also why they are overbuilt...to handle those extremes and compression.
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