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.

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.

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. ;)

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.

half the fuel at idle??
 

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

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

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.

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...

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.

https://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.

I would like to add one more answer to the many great ones here on why a diesel is so much more efficient at idle.

In my thinking the one major reason why a diesel is so efficient at idle (and low loads) is the way it ignites the fuel.

A gas engine ignites the fuel by creating a tiny spark in the cylinder. This spark ignites the fuel near it and spreads as a flame front through the cylinder until most of the fuel has ignited.

This way of igniting the fuel has a major drawback. The fuel to air ratio has to be high enough or the flame front will not spread in the cylinder.

That means when a gas engine idles it has to add a lot more fuel than what would energy wise be needed to turn the engine around just to make the ignition process work.

The diesel engine on the other hand ignites the fuel by creating a very high pressure in the cylinder. The high pressure makes the temperature in the cylinder so high that the fuel will ignite by itself as soon as it's sprayed into the cylinder.

Because the fuel in a diesel is self ignited by the heat of the air there is no need for a specific fuel to air ratio to make it burn, you just pump in as much fuel as is needed to turn the engine around.

I would call this THE MAJOR REASON why a diesel is efficient at idle.