Etienne76

Hi all,

Following a recent post I've read, I think that summarizing some basic knowledge could be of some help

A diesel engine is an engine that is working following the rule: "constant pressure pressure". A gas engine runs under the rule "constant volume combustion"

This is what makes the big difference, and what has to be done to get the more of it.

In a gas engine, once the mix air/gas is in the cylinder, valves closed and piston at +/- upper position, the spark will burn everything "instantaneously". The piston has not the time "to move" because the combustion is very fast. You need to input into the cylinder the right amount of gas/air in order to have the proper combustion. So you can "rotate" the engine at whathever rpm, the engine will still be able to run, because the combustion is "instantaneous". The speed of combustion is not a limiting factor.

In a diesel engine, the combustion takes more time to accomplish. So the diesel is pushing the piston while burning. The resultant problem is that over +/- 4500rpm, the piston speed (downwards) is higher than the combustion speed. So the piston will not be pushed more rapidly. So it is not really possible to increase the rpm in a diesel engine. Look at all RPM indicators into diesel cars : red line arrives at 4000 - 4500 rpm.

For a given engine, if you want to get more power, you need either to increase the torque, or/and you increase the rpm. because power = (torque X rpm)

Generally speaking, on both type of engine, torque is related directly to the "filling" of the cylinder. The more you put gas/diesel and air, the more you will get torque.
Rpm is generally limited by the "mechanic" and the design of the engine. you might have it running at higher rpm, so you will have more power, but the mechanic will suffer.

In a diesel engine, you can't increase the rpm, because of the limit given by the speed of combustion. So the only way to increase the power is to increase the torque. So you need to put more diesel inside (injection parameters), and more air inside (turbo, intercoolers,...) More over, the better the combustion is, the more energy you will take out of the diesel. So you need to have it injected in a manner that the droplets will be very, very fine, so that the specific surface of the injected diesel will be very high, and the combustion will be very easy and fast. To do this, the only way is to increase the injection pressure. Remember, years ago, the diesel was injected at 110 bars, now we're speaking about 1500+ bars. (Remark: the nice point in that, because they can reach very high injection pressure, so a very fine mist, the specific volume ratio was able to be reduced; a diesel engine like a 1600cc from 1980 was at 32 to 34 volume ratio, and now my 535d is at 16... so the engine is less suffering)

In a gas engine, you can increase both (torque and/or rpm) to get more power. but it is not as simple as in a diesel engine. because you need to have the right gas/air ratio in the cylinder when firing it...
Increasing the rpm is dangerous (to a certain point of course), because the mechanic will start to suffer; unless you start to reduce the moving weights, to equilibrate them....

hope this helps a bit

dxb_8788

To much to read

this way is easy
For more torque/hp - Do the following
for petrol engines add 500 shot(NOS) and turbo your 530i with T51r hks and boost it to 62 psi
this would help

Diamond

Yes some good info-what irritates me is how people misunderstand what torque and hp actually are.

Ultimately people seem to think torque is king and that is what "makes you go" and hp is "what keeps you going." which is the farthest explanation from the truth you can get.

Hp is the number you have to look at because it accounts for the 2 ways that make your car go-torque AND RPM's. Torque alone doesnt account for RPms which is why its an incomplete thing to look at.

A true statement is at low RPMS-such as going from a standstill-a car with Torque will pull harder through out the lower RPMS-because it is putting out an overall more HP in the low end.

As the RPMs climb-there is a point when the RPMs and the torque in a gasoline car will be combined enough to create equal/more horsepower.

So while it is true from a standstill or low revs more torque will launch you faster, once you get moving as long as you keep the RPMS nice and high in a fast spinning engine, there is absolutely no benefit to having more torque compared to less torque with high rpms

That is why M cars still pull hard in 0-60, quarter mile etc-because aside form the very beginning the revs get up high and stay up high.

People basically leave out RPM's in the equation and that is a key thing to factor in when comparing deisels and gasoline

response_surface

its very simple

torque = (Newton*meters)= mass*length^2/time^2 or kg*m^2/s^2

power = (joules/second ....aka watts) = mass*length^2/time^3 or kg*m^2/s^3


see how the time unit is at the 3rd power in its exponent? hence the speed of rotation has great effect on the total.

last time i posted a thing like this i got flamed like hell. at that time it was having to do with drive train losses.... power at the flywheel vs at the wheels and how to quantify those values, etc, etc.

so flame away!!!!!

healthservices

It is funny how everyone sees it a different way. A gallon of diesel fuel contains 18 percent to 30 percent more energy than gasoline.

In other words more energy per gallon means less fuel require to go the same mile. There is a reason the majority of Semis in a america are diesel.