What is Horse Power?
The power, in simple terms, is ‘rate or speed of doing work’. The work is defined as a 'force operating thru' a distance'. In terms of automotive engineering, it is 'horse power' (hp). The term ‘horse power’ comes from the erstwhile practice of measuring the rate of work done by a horse, which is equal to 33,000 ft-lb of work done per minute.
1 horse power (mechanical) = 33,000 ft·lbf/min = 550 ft·lbf/s
A conventional engines produces its power by burning the fuel. The combustion process releases the heat energy in the fuel, which results in generation of expanding forces known as power. The engine’s power is commonly measured in Brake-Horse-Power (bhp), Pferdestärke (PS) – which is the German translation of horsepower, kilowatt (kW) in case of metric system and in case of imperial / British system it is 'feet-pound-force-per-second' (fps).
Engine loses some of its generated power, while overcoming its friction (known as Frictional Horse Power). The Brake Horse Power, is the available power; which is almost about 70 to 85 % of the actual power developed inside the engine; known as the 'Indicated Horse power'.
You can calculate the Indicated Horse Power of a 4-stroke engine by the following formula -
I.H.P = ((PLAN x n) / 2) ÷ 4500
(P = Mean effective pressure in kg/cm², L = Length of stroke, A = Area of piston in cm², N = RPM of crankshaft, n = Number of cylinders)
The relation between the BHP, Ps & kW is:
1 Hp = 1.01 PS = 0.70 kW
1kW = 1.34 Hp = 1.4 PS
1 PS = 0.98 Hp = 0.70 kW
You can calculate the power output, if you know the torque value & engine rpm by the following formula.
Hp = Torque X RPM ÷ 5252
A dynamometer measures the power output of an internal combustion engine. Most automotive engines produce power in a wide band of the engine’s speed. Although, the power output varies from engines to engines; it is characteristically highest at the top-end of the engine’s rpm range. The power output of an engine, is usually represented in the graph; as a curve against the engine rpm band.
How does the horse power help?
The power curve in an engine, represents its rate of doing work; corresponding to engine’s rpm band. Higher the power an engine can produce, means; that the engine can do more work OR give higher performance. To achieve higher vehicle speeds, requires more power. The power is most useful while speeding & overtaking. More power is often needed when the vehicle is climbing a gradient, as the engine has to pull the vehicle’s weight against the forces of gravity.
What is ‘Power-Band’?
The engine speed range between ‘peak-torque’ rpm and ‘peak-horse power’ rpm is called the ‘Power-Band’. It is the range, in which; the engine is ‘most efficient’ & produces its greatest performance. Remember, the fuel consumption increases with the the engine speed. But, it is usually at the lowest within the engine’s ‘Power-Band’. Hence, always know the ‘power-band’ of your engine. If you change the gears within the ‘power-band’, you are likely to get the best fuel efficiency from your vehicle. To know about the ‘power-band’ of your engine, read the ‘Engine Specifications’ in Owner’s Manual. (Note: This range varies from vehicle to vehicle)
Which factors affect the engine’s horse power?
The engine power output varies from engines to engines and it mainly depends on the following factors - Engine design, Engine capacity, Type of fuel - petrol / diesel, no. of valves, Valve timing, Air charging - Naturally Aspirated / Turbocharged / Supercharged and Fuel Injection methods – MPFi / GDI / CRDi etc.
How to use engine’s horse power effectively?
The engine horse power, can best be used; in collaboration with the engine’s torque and, by changing gears accordingly. To get more speed, you need more power and to get more power, you need to burn more fuel. A conventional engine produces more power at the cost of higher fuel consumption.
Every engine reaches its Max. Power value, when raised to the full throttle; before the ECU cuts off its fuel supply. Every manufacturer mentions this value in the Owner's Manual, under the ‘engine specifications’, for e.g. ‘Maximum Power: 97 bhp at 5000 rpm’ as shown in the 2nd diagram (Engine Power Graph). The engine needs to produce maximum power & spin faster when speeding or overtaking. However, while cruising on a plain road; the engine need not to be run at max. power / rpm.