Torque reduces gradually after certain RPM

Question

I just know the plain definition of the Torque.

1. Torque: Definied as the force applied to the object to rotate on its axis
2. The torque will be increased only upto a certain RPM and it will gradually reduce.
3. For initial pickup

Q1: So, is this because, after a cetain RPM, the movement of the piston is very fast and so,it struggles to breathe the air from the environment, so in the power stroke, it is producing very less power?

Q2. If so, the engine with RAM intake, will assist the engine to breathe, also at the higher RPM.?
Q2.1 So, this results in producing the torque even at higher RPM.?

Q3. Also, this torque will depends on the quality of the air ( I mean the less denser,more denser with the oxygen molecules)...?

Is that right, or some other reasons are there..

Kindly help me, by providing some inputs for better understanding.

Thanks in advance...

Answer 1

Torque is a rotational force. It is a measurement of how hard it is applying force that causes the crankshaft to rotate.

Torque is not a very good unit of measurement for an engine because it does not take into consideration how fast the output shaft rotates. For example, given an engine with 100 lb-ft. of torque, if you put that output through a 10:1 gearbox, the gearbox output has 10x the torque (1000 lb-ft), but rotates 1/10th the speed it did without the gearbox.

Diesel engines operate at very low engine speeds (compared to a gasoline engine), so to make power, they need very high torque. Diesel engines benefit from lower wear from lower engine speeds, but they have very high compression, offsetting that benefit somewhat.

Power

If you take both the output shaft rotation speed and the torque into consideration, you end up measuring power. Power is measured in horsepower (in metric, watts).

Horsepower = torque (ft-lb) * RPM / 5252

Watts = N/m * rads/sec

The above are the same, the latter being metric.

Volumetric efficiency

The engine "struggling" to intake air at high engine speed is measured as "volumetric efficiency". It is the ratio of how much air actually fills the cylinder, to, how much air could fill the cylinder, at a given engine speed. It is usually expressed as a percentage.

Higher engine speeds should give you more power, automatically, because more fuel is burnt and more power strokes occur. This is true, but it is largely offset by significantly increased internal engine drag at high speeds.

Gearing

Consider a racing engine. If it can operate up to very high engine speed, it can stay in a lower gear longer. The lower gear gives them additional torque at the output of the gearbox, in exchange for spinning slower.

Conversely, a transport truck engine can hardly operate very far above 3000 RPM, it would need to up-shift much sooner.

Let's make a hypothetical comparison

100 lb-ft @ 8000 rpm vs 400 lb-ft @ 2000 rpm

Both of the above engines make exactly the same amount of power. In fact, you could make two identically powered output shafts, rotating at the same RPM, if you put the 8000 rpm engine through a 4:1 gear reduction, giving 400 lb-ft on both outputs.

You could also put the 2000 rpm engine's output through a 1:4 ratio. In this case, you would have 100 lb-ft on both.

Air Density

Yes, low air density (usually caused by high altitudes) will reduce engine output significantly. For this reason, aircraft engines are often fitted with turbochargers, to offset the lack of atmospheric pressure.

Ram air intake

Ram air intakes do have a small effect, but they are often there for psychological reasons. It doesn't have to actually help, it has to look cool, and it has to seem like it helps.

Answer 2

Most of my answer will be closely related to this answer: BSFC (Brake Specific Fuel Consumption) - How is this a beneficial measurement?

Q1. The force in each stroke is directly related to the torque. There is less force in each stroke as the torque is decreasing. Power will increase with RPM to a certain point as power is a function of force over time.

Q2a. If your limitation is air, a RAM intake will help minimally. The RAM air's main advantage is getting cool air from outside the vehicle, instead of hot air from under the hood. The RAM intake does not force much air (if any at all) when compared to a turbo or supercharger. You can look at the dyno graphs for a naturally aspirated and turbocharged engine (on the same bore, stroke, cam) and they will peak and drop around the same area.

Q3. Higher air density (colder air, close to sea level) produces more torque. Less dense air (hotter, higher altitude) produces less torque. This is because of the number of molecules or air being sucked into the cylinder. Its easy to match fuel on an electronic fuel injection engine, the hard parts is getting the air in.

Where your engine makes the most torque, and how wide the torque curve is depends on the design of the engine. I will touch on some main points.

Stroke - This is the most important to determining your power band. Longer stroke means more low end power. Shorter stroke means more high end power. Longer stroke gives more mechanical advantage (leverage) between the piston and the crank, so your torque peak will be higher. With a longer stroke, the piston has to more further and therefor faster for each stroke.

Cam - higher lift and more duration for higher torque curve. Most cars nowadays have some sort of variable cam that allows different profiles for low and high RPM.

Intake - long thin runners for low RPM, short fat for high RPM. Some cars have a variable intake.

Cylinder head - basically a match of the cam and intake. Larger valves for higher RPM. DOHC favors higher RPM. Larger port size for higher RPM, smaller port size for lower RPM.