3) Engine Rating

Automotive Engines are rated in terms of ‘Horse Power’ or eqvt ‘PS’ or ‘KW’. This can be defined or measured in more ways than one – such as:

i) Indicated HP (IHP)
It is the ‘theoretical’ Power that an engine can develop by the expansion of gases inside its cylinders, such as by means of ‘indicator cards’.

ii) Brake HP (BHP)
It is the Power that an Engine can deliver for ‘useful’ work, i.e. at its Flywheel and is measured on ‘Test-Beds’ using ‘Pony Brakes’ or ‘Dynamometers’.

For an Automotive Engine, it’s always expressed as a ‘peak/max value’ at a particular RPM at which it occurs and can be sustained for ‘some time’. None of the ‘Stock Car’ Engines can sustain this max Bhp ‘continuously’ as they are not designed accordingly.

In other words, a 100bhp/max car engine may be delivering ~ 70 bhp while cruising at 150kmph. The balance 30 bhp is the ‘reserve power’ needed for short duration over-taking or going up a small gradient without having to shift gears.

iii) RAC Rating
This was the ‘formula’ used by the ‘Royal Automobile Club’ of U.K. in the early 20th Century motoring days, mostly for Taxation purposes. It was ‘defined’ as:

1.0 (RAC) HP = D x D x N/2.5, where ‘D’ is the bore-diameter of the Cylinder in inches and N = the number of Cylinders a Car has. Thus an M-800 having a Bore dia of say 2.5” will have an RAC Rating of 7.5 HP!

However, the above formula was based on the assumptions that - a) the Mech. Efficiency of the Engine was 75%, b) Mean effective cylinder pressure was 90 psi and c) Piston speed was 1000 ft/min.

But now a days, these figs. have since got ‘upgraded’ to 85%, 130-150 psi and 2000 ft/min. Based on these, the old RAC rating would work out to 1/4th of the present one. In other words, the present day M-800 would have an RAC rating of 30 hp!!

iv) S.A.E Rating
For the purposes of standardisation, the SAE ‘defines’ the BHP of an engine as per (ii) above but when run without any ‘Auxiliaries’ such as the Radiator Fan, Air Cleaner and the Dynamo/Alternator. However, the Fuel, Lube oil and Jacket Cooling Water Circulation Pumps stand included.

The BHP so measured by a Dynamometer stands further corrected to ‘Normal Temperature, Pressure and Humidity conditions (NTP) and the value so obtained is known as the SAE-BHP Rating.

v) DIN Rating
This as defined in the German Standard and is measured/corrected to NTP conditions BUT includes all the Engine Auxiliaries otherwise excluded in the SAE definition.

In Part–II that follows, we’ll talk about the real life implications of an Engine’s ‘Speed/Torque/Power’ Characteristics.