Engine Horse Power

Horse Power: What is Horse Power? How to calculate and use it effectively?

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 automotives, 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

Engine Horse Power
Engine Horse Power

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 dynamo-meter 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.

Engine Horse Power graph diagram
Engine Horse Power graph diagram

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 also 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 ‘max-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)

Engine Power Band
Engine Power Band

Which factors affect the engine’s horse power?

The engine power output varies from engines to engines which 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 you need not run the engine at max. power / rpm.

Remember, the engine running at maximum rpm will only result in burning of additional fuel. So, use the accelerator sparingly to control the engine power and get the best efficiency out of it.…!!!


Read on: What is engine torque? characteristics,definition & formula >>

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