Car engine: how to calculate power and torque?

[JaNN]

Hello, everybody. I'm in trouble. I have to create program calculating the power and torque four stroke car engine. Unfortunatey, got no formulas. Could you help me, giving those formulas with any describition of them? And, have you got any other advice? Please, it's very urgent.
Yours,
JaNN

 

[berkeman]

Here you go:

http://auto.howstuffworks.com/question622.htm

You could probably also get a lot of good hits from a google search.

-Mike-

 

[Josh Durbin]

All of the websites say that you need a dynamometer to calculate torque, and then from the torque curve produced, you may calculate the hp curve. However, i don't have access to a dyno... is there any way possible to calculate the hp of an engine using mathematics? I know the max torque and the rpm at which it occurs as given by the car specs, i found the drag, and also the tire resistance.. then i used that data to come up with a hp formula, but this is only when the car is in 5th gear.. however, my formula is an x^3 graph, not a logarithmic looking graph like the ones professionally done... so is it possible to make a professional one, without using a dyno? or am i just dreaming?

 

[rcgldr]

An engine dynamometer measures torque and rpm, but a chassis dynamometer measures force and speed. Either can be used to calculate power:

power / horsepower = torque (ft lb) x rpm x 2 x pi / 33000 = torque (ft lb) x rpm / 5252.113122...

power / horsepower = force (lb) x speed (mph) / 375

Drivetrain losses are usually 13% to 17%, depending on the car.

Depending on where you live, you might be able to find a chassis dyno shop that can do a dyno run on your car.

 

[Bob S]

Here is an article by Marc Ross (U. Mich.) on the physics of automobile engines.
http://sitemaker.umich.edu/mhross/files/fueleff_physicsautossanders.pdf
Of particular importance is the BSFC (brake specific fuel consumption) plot showing engine efficiency plotted vs. RPM, measured on a flywheel dynamometer. (83 grams of fuel per kilowatt-hour is 100% efficiency, 330 grams is about 25%. Note that maximum efficiency is at about 35% of redline and 80% of maximum torque.

 

[xevilstar]

electric engine calculations

I have a 24 inches lever, an electric engine is at one end of it and it has to lift 30 pounds of weight on the other end. The engine end of the levere is fixed to the base of the machine, the other end is fixed on the base of the burden. I need to know the needed specs of the engine (rpm wattage and torque) so that I will not buy a wrong one.

 

[xxChrisxx]

You can start a new thread you know :)

Without using a dyno you will need to know the weight of the reciprocating components to build an inertia load vs crank angle.

You will also need a map of the pressures in the cylinder over 2 crank rotations (ie all 4 cycles). From this you can work out the pressure load vs crank angle.

This will give you the force acting down the cylinder axis for a given crank angle.

From this you need to convert the force ating down the axis into components at the crank (there is a 'turning' component used to create the useful torque, and a 'bending' component that tried to bend the crank).

You can do this for a range of rev values to find a very rough idea of power output.

WARNING: This method assumes a constant BMEP and will only be valid at RPM close to the BMEP it was taken at. You would need pressure vs crank angle map at multiple points to build a decent curve.

 

[xevilstar]

how can I get those datas from the ones I have provided ? can anyone give me the exact calculations needed ?

You can start a new thread you know :)

Without using a dyno you will need to know the weight of the reciprocating components to build an inertia load vs crank angle.

You will also need a map of the pressures in the cylinder over 2 crank rotations (ie all 4 cycles). From this you can work out the pressure load vs crank angle.

This will give you the force acting down the cylinder axis for a given crank angle.

From this you need to convert the force ating down the axis into components at the crank (there is a 'turning' component used to create the useful torque, and a 'bending' component that tried to bend the crank).

You can do this for a range of rev values to find a very rough idea of power output.

WARNING: This method assumes a constant BMEP and will only be valid at RPM close to the BMEP it was taken at. You would need pressure vs crank angle map at multiple points to build a decent curve.

 

[xxChrisxx]

how can I get those datas from the ones I have provided ? can anyone give me the exact calculations needed ?

I actually didn't answer your question, I answered the OP's. Please ignore the above, this is also why you should start a new thread for new questions it's avoids confusion like this.

I have a 24 inches lever, an electric engine is at one end of it and it has to lift 30 pounds of weight on the other end. The engine end of the levere is fixed to the base of the machine, the other end is fixed on the base of the burden. I need to know the needed specs of the engine (rpm wattage and torque) so that I will not buy a wrong one. /QUOTE]

torque = force * length
This is the minimum torque needed to move the item.

Power and RPM (they are connected) required depends on how quickly you need to lift it.
Power = torque * angular velocity

The quicker you need to lift the item the more angular velocity you will need.

 

[Greg Bernhardt]

Power is the rate at which work is done. It is the product of force and velocity. Basically, it is how much work is done in a given amount of time. Sometimes, power is calculated as force multiplied by distance divided by time (P = F × d / t). For example, if you are lifting a 5-kg weight off the floor, your force is 5 N. If you are lifting it to a height of 1 m in 1 s, your power is 5 kg m/s.

Torque is a measure of the turning force applied to a shaft or axle. It is equal to the product of the force applied and the perpendicular distance from the line of action of the force to the axis of rotation. T = F r Torque is also known as moment and is measured in Newton-metres (Nm) and kilogram-centimetres (kg-cm). The SI unit for torque is the Newton metre (Nm). The older unit of pound-foot (lb.-ft.) is still commonly used. The direction of torque is the same as the direction of the force. A force applied at right angles to a lever arm produces a torque which is equal to the product of the force and the length of the lever arm. T = F x L Where F is the force and L is the length.