The reason diesels are efficent at idle? Solved!

[landspeed]

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).

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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?

[omgwtfbyobbq]

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.

[Bill in Houston]

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....

[Hockey4mnhs]

i also can see that

[omgwtfbyobbq]

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. :thumbup:

[landspeed]

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.

[omgwtfbyobbq]

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.

[usedgeo]

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:rolleyes: .

[Mike T]

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.

[DarbyWalters]

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.

[91CavGT]

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.

You beat me to it!


Most if not all diesels also have longer strokes than bores (I can't remember if this is over square or under square). With how slow diesel burns the motor needs the longer stroke to get as much power out of the air/fuel mix as possible when it is combusted. Diesels also run very high fuel pressures (some as high as 20,000 psi) which help to atomize the fuel better to ensure a full burn. These in combination help to make a diesel capable to running at extremely lean air/fuel ratios without blowing the motor.

[bobski]

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.
While you have the right idea, it has very little to do with the number of cams the engine has. DOHC designs simply make it easier to run 16 (or 20 in some Toyota designs) valves and place the spark plug in the center of the cylinder. I've got a 16 valve SOHC engine in my CRX, so two cams is not a requirement for a 16 valve 4-cylinder.
Assuming the intake and exhaust are designed properly, power and torque curves are decided primarily by the profile of the cam lobes. That is, when the valves open and close and how far they open. Some recent advancements in engine technology allow the ECM to adjust the cam profile on the fly. Toyota calls it VVT-i (Variable Valve Timing with Intelligence) if I'm not mistaken... Honda calls it VTEC (Variable valve Timing and lift Electronic Control). Toyota's first VVT engines were DOHC, with a cam gear that included a hydraulic mechanism to adjust the cam phasing... It changed the timing of the (intake only I think) valves opening and closing depending on oil pressure (which is dependent on engine RPM). The change in timing varies the amount of valve overlap (the period of time between the exhaust and intake strokes where both the intake and exhaust valves are open) present at a given engine speed, optimizing cylinder scavenging for a given engine speed.
Honda's VTEC adds an extra cam lobe and rocker (or two in DOHC engines - 1 intake, 1 exhaust) between the regular rockers for each cylinder. The extra lobe has a profile that makes good power at high RPM, while the normal lobes make good low-end power. When VTEC is flipped on (generally above 5k RPM), oil pressure forces a pin to link all 3 rockers so they follow the high RPM cam profile. Using a completely different cam lobe allows for adjustment of valve timing, duration and lift, but it's limited to on/off operation.
AFAIK, Toyota and Honda have taken some of eachother's ideas into their current engine designs (VVTL-i and iVTEC) by integrating both multiple cam profiles and variable cam phasing.

[omgwtfbyobbq]

I wouldn't even go as far as to say it's related to the number of valves, since all the 4 valve SOHC engines I've heard of are interference engines as well. Apples and oranges and alla that... What I'm saying is given the engines I was referring to, there's no way a non-interference SOHC engine could have as broad of a torque curve as a DOHC version of the same engine, all things being equal, which they rarely are. ;)

[Nerobro]

An easy way of "grasping" why diesels are more economical has been covered. Diesels run at full throttle ALL the time, so they are as thermally efficant as they can be. They need to burn very little fuel at 20:1 to extract enough energy to keep the motor turning.

When you are at part throttle on a spark ignition motor, you are running a motor at 2:1 or less for the compression ratio. That's deep off the bad end of the CR:Power curve. So you need to burn more fuel to keep the motor running.

Now, at part throttle, if you could change the compression ratio of a gas engine on the fly... you could get good power for a given amount of fuel despite being at part throttle. ... Saab has this technology. It's not simple, but it works.

Diesels can burn so freaking lean becasue they don't need to ignite the fuel mixture. A diesel works by shooting fuel into a hot space. It's much the same as shooting a can of wd-40 over a match. The fuel finds it "own" air. This is a very slow way to burn fuel. Which limits the maximum rpm of diesel motors. (diesels don't turn slowly becuase they're built heavy, they turn slow because they can't burn fuel over 5200 rpm or so)

Gas motors get around that by mixing the fuel and air, so the flame front can travel throughout the whole combustion chamber, without having to wait for the fuel to reach the flame front.

Turning slow also reduces pumping losses. You can get the same, or nearly the same benifits with a gasoline motor. At least if you don't try to run it at idle. Which is the whole point of this thread isn't it?

Now if you're talking about making power. Things get even more funny. A diesel can only burn roughly 2/3 of the air in the combustion chamber, because that flamethrower I was talking about, can't shoot fuel far enough, fast enough, in a proper pattern to use all the air. That's why turbochargers are the bees knees on diesels. By stuffing more air in, you can burn more fuel. And because diesels aren't operating with a potentially explosive mix in the combustion chamber, you don't get pinging.

Gas motors are under less stress more of the time. They can be built lighter. Their ancillary equipment is lighter. They are smaller for the same amount of power, or at least cheaper. Diesels need turbos to make the same power in the same size package.

IT's real food for thought.

[dddon]

this is a an off the wall question for you guys ...
if you really are trying to cut fuel use at idle .. has anyone tried switching off half of the injectors?? i picture the ground wires run to a relay or switch ... i used to work a lot on VW air cooled 1600 cc engines ... they would run on two cylinders, or if it was a good engine it would run on one!! not smooth.... but hey ..with injectors it could mean less fuel burned?? dddon

[RoadWarrior]

dddon we've got a few threads on that, variable displacement, displacement on demand etc.

[Powerstroke IN]

The reason diesels are efficent at idle is because they are only putting enough fuel into the cylinder to overcome internal friction. That's why they sound liek they're about to die... They are. Diesels only introduce the fuel required to make the combustion called for by the rpm. Their air/fuel curve isn't dictated by a throttle, but rather the computer saying, "this much pressure is in the go button, therefore this much fuel and this much boost need to be acheived to get the correct fire. Meanwhile, gasoline engines have no computer to tell them how much fuel to introduce, therefore they're still pumpin' fuel in to match the O2 levels. Not to mention that a diesel engine gets about 15-20% more energy from it's fuel that gasoline. Let's face it, diesel is best and always will be.

[Powerstroke IN]

You beat me to it!


Most if not all diesels also have longer strokes than bores (I can't remember if this is over square or under square). With how slow diesel burns the motor needs the longer stroke to get as much power out of the air/fuel mix as possible when it is combusted. Diesels also run very high fuel pressures (some as high as 20,000 psi) which help to atomize the fuel better to ensure a full burn. These in combination help to make a diesel capable to running at extremely lean air/fuel ratios without blowing the motor.

My fuel pressure gauge read 35,000 PSI at full throttle, but my engine has been heavily, heavily modified. And the boost gauge reads 62-64 PSI. It's a glorious thing to hear...

[fumesucker]

Stroke longer than bore is undersquare and bore larger than stroke is oversquare.

One point that hasn't been directly addressed is cylinder pressure.. At an idle the cylinder pressure in a diesel is much greater than that in a spark engine.. When you compress a gas, it gets hot, the greater compression in a diesel, particularly at idle, leads to a hotter gas at TDC on the compression stroke. Since the intake mixture in a diesel is already hotter and denser than the mixture in a spark engine then the additional heat provided by burning the fuel boosts temperature and pressure even higher.

Since all engines which run on heat are subject to the Carnot cycle limitations, the higher the heat and pressure at the hot end of the cycle (TDC, compression stroke) then the higher the Carnot efficiency can be.

http://en.wikipedia.org/wiki/Carnot_cycle

To recap, there are three major reasons why a diesel is more efficient.. Minimal pumping losses due to no throttle plate, extremely lean mixtures at idle which efficiently burns the diesel fuel and, last but not least, higher pressures and temperatures at the hot/compressed end of the Carnot cycle.

If you do not understand the Carnot cycle then you basically are flying blind in trying to modify an internal (or external for that matter) combustion engine for greater efficiency.