# Calculating Engine Characteristics

1. Nov 2, 2008

### get2kool

I need to figure out how much power and torque my engine has to produce to move a vehicle at 25mph. Can you tell wat all factors i have to consider and the formulae to calculate them?Weight of the vehicle is 200kg. tire width is 90mm and dia = 16"

2. Nov 2, 2008

### brewnog

Moving at a constant speed of 25mph needs enough power to overcome aerodynamic drag, rolling resistance of the tyres, and mechanical losses in the drivetrain.

Aerodynamic drag is calculated using F = 0.5*p*v^2*A*C, where p (is supposed to be a rho!) is density, v velocity, A the frontal area of the body, and C the drag coefficient. Use SI units.

Rolling resistance is much more difficult to calculate accurately, but a reasonable approximation can be given by F = C*N, where C is the rolling resistance coefficient, and N the normal force applied.

Mechanical losses are very difficult to calculate, and an empirical estimation would be safest.

Power is force x velocity, so in this case, the power required to move at a constant 25mph is the sum of all the forces resisting motion multiplied by the body's velocity.