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13-12-2004, 10:16 PM
To enable intelligent comparisons to be made between different engine's ability to pull or operate at various speeds, we shall now consider engine design parameters and there relationship in influencing.
2-2-1 swept volume:
When the piston moves from one end of the cylinder to the other, it will sweep or displace air equal to the cylinder volume between TDC and BDC. Thus the full stroke movement of the piston is known as either the swept volume or the piston displacement.
The swept volume may be calculated as follows:
V= πd2 L
4000
Where:
V: piston displacement [cm3]
Π: 3.142
d: Cylinder diameter [mm]
L: Cylinder stroke [mm]
2-2-2 Mean effective pressure (MEP):-
The cylinder pressure varies considerably while the gas expands during the power stroke. Peak pressure will occur just after TDC, but this will rapidly drop as the piston moves towards BDC. When quoting cylinder pressure, it is therefore more helpful to refer to the average or mean effective pressure throughout the whole power stroke. The unites used for MEP may be either [KN/m2] or bar.
2-2-3 Engine power and torque:-
In the automotive world, lots of things move in circles rather than in straight lines. Think of the wheels or engine. Some of the above concepts behave differently and use different names. However, they relate to one another in the same way. Distance is no longer feet or meters, it is revolutions measured in radians. Speed is revolutions (or radians) over time instead of distance over time, for example, revolutions/minute (RPM) or radians/sec. Mass is called inertia. We still need some kind of "push" to get something spinning. Instead of force, we have torque which is "twisting" force so to speak. Like force, torque must be applied to get something to spin faster or slower (accelerate).Torque is measured by the amount of force applied tangentially at a given distance and is given units of force * distance. In the metric system, we would use Newton*meter (N*m). In the US, we would use pound*feet (lbs*ft). For some strange reason we usually say "foot pound" instead of "pound feet" but use both lbs*ft and ft*lbs. Think of a wrench. We pull on the handle to turn the bolt. The longer the wrench, the more torque exert on the bolt with the same amount of force.
As mentioned above, work is force*distance. The major source of confusion about torque comes from the units. Torque in lbs*ft or N*m has the same units as work. Work is energy, torque is a force.
Work in the rotational world is still basically force*distance. Torque is our force and distance is our revolutions in radians (1 revolution = 6.28 radians roughly). We use the same units such as lb*ft, N*m, or J. The reason the units do not change is that the distance (radians) is a dimensionless quantity. Don't worry about it. Just know that you have to say x lbs*ft of torque or x lbs*ft of work or energy to differentiate them.
Similarly, power is work/time or force*velocity. Since work in the rotational world has the same units as in the linear world, power has the same units also. We would use lbs*ft/s, J/s, W, or hp for example.
To get our vehicles moving, we use some kind of engine to convert chemical energy (fuel) into useful work. In an internal combustion engine, the fuel in the combustion chamber is ignited by a spark (gasoline) or from compression (diesel), creating high pressure gasses that press down on the pistons. Via the connecting rods, this force pushes the crankshaft throws or arms. Bearings control the crankshaft, allowing it to only rotate. Since each crank arm is offset from the centerline of the crankshaft (the rotational axis), we have torque.
So far we have been kind of simple. The combustion gases develop pressure right away and continue while the crank is turning. Hence, sometimes the force along the connecting rod is not tangential to the crank rotation; it is that way only instantaneously. To further complicate matters, the crank turns at varying rates (engine rpm), different amounts of gasses expanding (throttle), etc. The bottom line is that engines develop differing amounts of torque during operation which is hard to predict. However, we can easily measure it.
If we assume that we want maximum torque figures, we assume maximum throttle. All we then worry about is how much torque at what rpm. We end up with a curve. Engine design, fuel systems, etc all change the shape of the torque curve. Note that in most cases, torque rises to some peak value and then decreases.
So what about power? If we remember from above, power is torque*distance/time or is torque*velocity. For this discussion, we want to take torque in lbs*ft and speed in revs/minute and calculate horsepower:
Horsepower = torque * revs/minute * minute/60 s * 2*pi * 1/550
Horsepower = torque * revs/minute * 1/5252
In the first line, torque is in lbs*ft, speed in revs/minute. The third term just converts RPM to revs/s. The fourth term converts revs to radians. The last term is the conversion factor from lbs*ft/s to horsepower. The second equation just multiplies out the constants to make life easy.
Using the above equation, we can add a horsepower curve to our torque graph. Note that the horsepower peak is at a higher rpm than the torque peak.
Torque and horsepower are very closely related. We can see that a high revving motor does not need to produce much torque to get lots of horsepower. Similarly, a high torque engine may not develop lots of horsepower.
2-2-4 Load:-
One thing that should be considered is that the torque and power graphs illustrate the maximum levels possible. An engine can develop less than that amount at a given rpm. The throttle controls the amount of fuel to the motor and, hence, the torque and power produced.
Earlier, we said that force and torque cause acceleration. If we are at a constant speed, how come we continue to need power or torque? The answer is friction or drag. The drag torque tries to slow us down so we add an opposite torque to prevent the deceleration. So in the case of steady speeds, we only need the engine to produce enough torque to compensate for the drag torque.
When idling, all the power goes to overcome the drag from friction (assuming no accessories). You can barely touch the throttle to rev the motor 2000 rpm. However, when cruising down the highway at 2,000, you are using much more power to overcome friction, rolling resistance, wind drag, etc. At top speed, 100% of the torque goes to overcome drag.
2-2-5 Engine cylinder capacity:-
Engine size is compared on the basis of total cylinder swept volume, which is known as engine cylinder capacity. Thus the engine cylinder capacity is equal to the piston displacement of each cylinder times the number of cylinder.
VE = V*n / 1000
Where:
VE: Engine cylinder capacity [Litter].
V: Piston displacement [cm3].
n: number of cylinders.
Piston displacement is derived from the combination of both the cross sectional area of the piston and it's stroke. The relative importance of each of these dimensions can be demonstrated by considering how they affect performance individually.
The cross sectional area of the piston crown influences the force acting on the connecting rod, since the product of the piston area and the mean cylinder effective pressure is equal to the total piston thrust:
F = P * A
Where:
F: Piston thrust [KN].
P: Mean effective pressure [KN/m2].
A: Cross sectional area of piston [m2].
The length of the piston stroke influences both the turning effort and the angular speed of the crank shaft. This is because the cark throw length determines the leverage on the crank shaft, and the piston speed divided by twice the stroke is equal to the crank shaft speed:
N = ν / 2L
Where:
N: Crank shaft speed [rev / min].
ν: Piston speed [m / min].
L: Piston stroke [m].
This means that making the stroke twice as long doubles the crank shaft turning-effort and halves the crank shaft angular speed for a giving linear piston speed. The above shows that the engine performance is decided by the ratio of bore to stroke chosen for a giving cylinder capacity.
2-2-1 swept volume:
When the piston moves from one end of the cylinder to the other, it will sweep or displace air equal to the cylinder volume between TDC and BDC. Thus the full stroke movement of the piston is known as either the swept volume or the piston displacement.
The swept volume may be calculated as follows:
V= πd2 L
4000
Where:
V: piston displacement [cm3]
Π: 3.142
d: Cylinder diameter [mm]
L: Cylinder stroke [mm]
2-2-2 Mean effective pressure (MEP):-
The cylinder pressure varies considerably while the gas expands during the power stroke. Peak pressure will occur just after TDC, but this will rapidly drop as the piston moves towards BDC. When quoting cylinder pressure, it is therefore more helpful to refer to the average or mean effective pressure throughout the whole power stroke. The unites used for MEP may be either [KN/m2] or bar.
2-2-3 Engine power and torque:-
In the automotive world, lots of things move in circles rather than in straight lines. Think of the wheels or engine. Some of the above concepts behave differently and use different names. However, they relate to one another in the same way. Distance is no longer feet or meters, it is revolutions measured in radians. Speed is revolutions (or radians) over time instead of distance over time, for example, revolutions/minute (RPM) or radians/sec. Mass is called inertia. We still need some kind of "push" to get something spinning. Instead of force, we have torque which is "twisting" force so to speak. Like force, torque must be applied to get something to spin faster or slower (accelerate).Torque is measured by the amount of force applied tangentially at a given distance and is given units of force * distance. In the metric system, we would use Newton*meter (N*m). In the US, we would use pound*feet (lbs*ft). For some strange reason we usually say "foot pound" instead of "pound feet" but use both lbs*ft and ft*lbs. Think of a wrench. We pull on the handle to turn the bolt. The longer the wrench, the more torque exert on the bolt with the same amount of force.
As mentioned above, work is force*distance. The major source of confusion about torque comes from the units. Torque in lbs*ft or N*m has the same units as work. Work is energy, torque is a force.
Work in the rotational world is still basically force*distance. Torque is our force and distance is our revolutions in radians (1 revolution = 6.28 radians roughly). We use the same units such as lb*ft, N*m, or J. The reason the units do not change is that the distance (radians) is a dimensionless quantity. Don't worry about it. Just know that you have to say x lbs*ft of torque or x lbs*ft of work or energy to differentiate them.
Similarly, power is work/time or force*velocity. Since work in the rotational world has the same units as in the linear world, power has the same units also. We would use lbs*ft/s, J/s, W, or hp for example.
To get our vehicles moving, we use some kind of engine to convert chemical energy (fuel) into useful work. In an internal combustion engine, the fuel in the combustion chamber is ignited by a spark (gasoline) or from compression (diesel), creating high pressure gasses that press down on the pistons. Via the connecting rods, this force pushes the crankshaft throws or arms. Bearings control the crankshaft, allowing it to only rotate. Since each crank arm is offset from the centerline of the crankshaft (the rotational axis), we have torque.
So far we have been kind of simple. The combustion gases develop pressure right away and continue while the crank is turning. Hence, sometimes the force along the connecting rod is not tangential to the crank rotation; it is that way only instantaneously. To further complicate matters, the crank turns at varying rates (engine rpm), different amounts of gasses expanding (throttle), etc. The bottom line is that engines develop differing amounts of torque during operation which is hard to predict. However, we can easily measure it.
If we assume that we want maximum torque figures, we assume maximum throttle. All we then worry about is how much torque at what rpm. We end up with a curve. Engine design, fuel systems, etc all change the shape of the torque curve. Note that in most cases, torque rises to some peak value and then decreases.
So what about power? If we remember from above, power is torque*distance/time or is torque*velocity. For this discussion, we want to take torque in lbs*ft and speed in revs/minute and calculate horsepower:
Horsepower = torque * revs/minute * minute/60 s * 2*pi * 1/550
Horsepower = torque * revs/minute * 1/5252
In the first line, torque is in lbs*ft, speed in revs/minute. The third term just converts RPM to revs/s. The fourth term converts revs to radians. The last term is the conversion factor from lbs*ft/s to horsepower. The second equation just multiplies out the constants to make life easy.
Using the above equation, we can add a horsepower curve to our torque graph. Note that the horsepower peak is at a higher rpm than the torque peak.
Torque and horsepower are very closely related. We can see that a high revving motor does not need to produce much torque to get lots of horsepower. Similarly, a high torque engine may not develop lots of horsepower.
2-2-4 Load:-
One thing that should be considered is that the torque and power graphs illustrate the maximum levels possible. An engine can develop less than that amount at a given rpm. The throttle controls the amount of fuel to the motor and, hence, the torque and power produced.
Earlier, we said that force and torque cause acceleration. If we are at a constant speed, how come we continue to need power or torque? The answer is friction or drag. The drag torque tries to slow us down so we add an opposite torque to prevent the deceleration. So in the case of steady speeds, we only need the engine to produce enough torque to compensate for the drag torque.
When idling, all the power goes to overcome the drag from friction (assuming no accessories). You can barely touch the throttle to rev the motor 2000 rpm. However, when cruising down the highway at 2,000, you are using much more power to overcome friction, rolling resistance, wind drag, etc. At top speed, 100% of the torque goes to overcome drag.
2-2-5 Engine cylinder capacity:-
Engine size is compared on the basis of total cylinder swept volume, which is known as engine cylinder capacity. Thus the engine cylinder capacity is equal to the piston displacement of each cylinder times the number of cylinder.
VE = V*n / 1000
Where:
VE: Engine cylinder capacity [Litter].
V: Piston displacement [cm3].
n: number of cylinders.
Piston displacement is derived from the combination of both the cross sectional area of the piston and it's stroke. The relative importance of each of these dimensions can be demonstrated by considering how they affect performance individually.
The cross sectional area of the piston crown influences the force acting on the connecting rod, since the product of the piston area and the mean cylinder effective pressure is equal to the total piston thrust:
F = P * A
Where:
F: Piston thrust [KN].
P: Mean effective pressure [KN/m2].
A: Cross sectional area of piston [m2].
The length of the piston stroke influences both the turning effort and the angular speed of the crank shaft. This is because the cark throw length determines the leverage on the crank shaft, and the piston speed divided by twice the stroke is equal to the crank shaft speed:
N = ν / 2L
Where:
N: Crank shaft speed [rev / min].
ν: Piston speed [m / min].
L: Piston stroke [m].
This means that making the stroke twice as long doubles the crank shaft turning-effort and halves the crank shaft angular speed for a giving linear piston speed. The above shows that the engine performance is decided by the ratio of bore to stroke chosen for a giving cylinder capacity.