[alpha1]

Originally Posted by Samurai

When you are idling, the A-pedal is not pressed, very little fuel is spent, just to keep the engine running. When you press the A-pedal at neutral, you will spend little more fuel to keep the engine running at higher rpm. The torque generated is just to keep the engine running, in both cases.

Once the gearbox is engaged, and car starts moving, fuel is being spent to do the extra work of moving the car, and even accelerating the car.

Less load means less torque and less fuel requirement. More load means more torque and more fuel requirement.

Each engine also has an upper limit.

Is the resistance to expansion stroke (aka load) constant?

The bold part is where I have my doubts, in fact it should be the other way round.

Apologies for the extreme verbose prose but I guess no other way to explain it. Summary: RPM is the manifestation of (im)balance of forces/torque/power (whichever you may prefer).

Case1: The engine is idling in neutral (say 700 RPM). Idling means it is not accelerating = it is generating enough power which is getting dissipated. If we introduce any less fuel and/or air the power produced is lower than friction & losses threshold and the engine dies.

Case 2: Still in neutral. Slightly depress accelerator pedal = slight more opening in the intake butterfly valve = slight more ingestion of fuel & air = slight more production of force/torque = slight acceleration = increase in RPM. Till it achieves another equilibrium between friction & losses and power being produced from this engine at this RPM (1000 RPM).

Case 3: Do the same as above, but at first gear. You will realize that in order to maintain the same RPM (1000 RPM), accelerator pedal needs to be depressed more compared to Case 2, which means more fuel has to be burnt to generate more power to balance against the new additional forces imposed by the movement of the vehicle (which is dependent on the speed of the vehicle).

Case 4: when you have achieved equilibrium in Case 3, press the clutch pedal completely, but keep the accelerator input unchanged. You will see a sudden acceleration in RPM (to say 3000 RPM). This is because the engine is still producing the same force/torque/power as before, but it doesn't face the same resistance at 1000 RPM (vehicle load has been taken out).
It accelerates till 3000 RPM where the friction & losses again balance against the engine power and provide an equilibrium, at which point the RPM steadies at 3000 RPM.

I am not denying that the load causes a resistance on the engine piston and that may change the whole delivery of force/torque/power character.
However, the primary observation that naturally comes to my mind is that engine inherently produces certain force/torque/power based on the fuel and air that is ingested and burnt.

Originally Posted by Jeroen

Neither the Force due to combustion is constant nor is the arm that creates torque constant. The crank turns around at 0o and 180o the arm is actually exact zero! So is the torque. At 90o past top dead centre the arm is maximum. On the expansion stroke this does not necessarily coincide with the most force exerted on the position due to the combustion/expansion.

So the amount of torque depends/varies depending on the crank position, how the combustion and expansion takes place and in which part of the two or four stroke cycle you are.

These varying torque values can create problems. Can cause vibration and undue stresses.

A flywheel is partly there to dampen out that effect. (Among other things). On larger engines you might also find dynamic balancers. ((E.g. my Jeep Cherokee)

On both petrol or diesel engines as a rule of thumb to calculate the force on the piston you need to look at the pressure in relation to the position of the piston. If you know the dimension of the piston and the pressure you can calculate the theoretical force. For practical purposes you need to also consider centripetal forces (depends on rpm, mass and stroke). Also, you need to add in the gravitational constant and the forces that it creates.

So it is a bit more complicated. But on both petrol and diesel engine you will find that the pressure in the cylinder rises during compression and peak during ignition, which usually start a bit before top dead centre and ends just after top dead centre. The pressure peaks and as the pistons moves downwards the pressure drops as well.

Jeroen, not going to dispute with what you have mentioned above, since it is definitely happening.

However, Case 1 to Case 4 illustrations above makes me feel that engine inherently produces certain force/torque/power based on the fuel and air that is ingested and burnt, rather than being mostly dependent on external load.


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Jeroen & Samurai, one thing I am fully in agreement with your points is that the published torque-RPM or power-RPM curves for an engine are done at wide open throttle and maximum possible brake load on the engine.

[venkyhere]

Originally Posted by alpha1

Jeroen & Samurai, one thing I am fully in agreement with your points is that the published torque-RPM or power-RPM curves for an engine are done at wide open throttle and maximum possible brake load on the engine.

In fact, the torque and bhp curves are the "envelope" of operating points of the engine. Every point in the graph located "under" these curves is a real world operating point of the engine. Hence, 99% of the time, our engines are running 'under' these curves, and on very rare 'highest load' situations , does it even get to sit 'on the curve'.

A fully laden or overweight truck will mostly spend its time near the envelope , ie sitting on or near the curves, whilst an empty truck will be mostly operating "far under" the curves.