We are all aware that today’s Cars are a far cry from the good old Ambys and Fiats of the ‘70s/’80s.

Here is the third and the last one of the ‘Q n A’ sessions spread over three ‘editions’.

Q18: What is PS? What is its relation with BHP?

A18: ‘PS’ stands for ‘Pferde Starke’ and it’s a unit of Power Measurement. It was popular post-war Germany and still in use there. One PS is slightly less than one HP (1 HP ~ 1.07PS).

Q19: What is a ‘Common Rail’ Diesel Engine? How does it work?

A19: A ‘common rail’ diesel engine does away with the ubiquitous diesel FI Pump, as we know it. It is a diesel fuel injection system where in an Engine or Electrically driven pump keeps a Rail/Header constantly pressurised to a uniform pressure of about 30-50 bars.

Each cylinder’s ‘fuel-injector’ is tapped on to such a header. The injectors in turn, like an mpfi Petrol Engine, are Electrically/Solenoid Operated, based on the EMS-Comp commands, and ‘squirt’ the diesel fuel at high pressure into the cylinders.

Such a system results in higher fuel efficiency/BHP and a smoother power delivery, compared to the conventional FIPs hitherto in use.

Q20: What is the ‘Compression Ratio’ of an Engine and how does the company fix it when they make the engine?

A20: The Power that an ICE can develop is given by the formula -
BHP = P x L x A x N, where P = the ‘Brake mean effective pressure’ in a cylinder during the ‘complete power cycle’, L = Piston Stroke, A = Bore Area and N = RPM. Further :

1) The 'CC' of an ICE is the max volume of Water a Cylinder of it can hold i.e. with the Piston at the 'bottom most of its Stroke' (BDC), multiplied by the number of cylinders it has. Therefore, the unit cyl vol of an M800 is 796/3 = 265 cc.

2) The 'Compression Ratio' (CR) of an ICE is defined as "Swept Volume/Clearance Volume". From (1) above, it may be inferred that CC = SV+CV.

3) Every ICE is designed and produced to have a CR as one of it's vital parameter, for max power/efficiency it can produce, GIVEN the type of fuel it's designed to operate with. In our Country, the "regular" Gas is 87 Octane and "premium" is 93. An M800 is, therefore, designed for 87 Octane with a CR of 8.7:1.

4) It may be noted here that higher the CR, the higher Octane Rating Fuel it would require to produce higher Power - for a given CC of the Engine.

Q21: What is the difference between an Alternator and a Dynamo as far as working, efficiency and performance is considered? Which is better and why?

A21: Any Rotating Electrical Machine is ‘inherently’ AC!

For DC applications, as necessitated in a Car due to the unavoidable existence of a Battery, which can only be DC, the Dynamos/Alternators/Starters all ‘have’ to be ‘converted’ to be ‘DC Friendly’.

In the Dynamos and Starters, it’s done by using an in-built ‘Rotating Mechanical Rectifier’, commonly known as the ‘Commutator’. On the other hand in an Alternator, it’s done by using a built-in solid state ‘Bridge-Rectifier’ system.

Talking of a Yesterday’s Dynamo, which was invented long before reliable and durable ‘solid-state/high-powered’ Rectifiers became commercially viable, the commutator part of it was the ‘black-sheep’, by way of reliability/durability and Radio-interference.

Besides, it needed an external ‘cut-out/voltage-regulator’ too, coz of its inability to develop sufficient voltage at idling Rpms and in any case when the Engine was to be shut down, it had to be isolated from the Battery as otherwise, the Battery would be short-circuited/discharged through it !

The mass availability of solid-state power electronics by the ’60 gave way to a lot more robust and reliable ‘Alternator’ for the Car applications, as the Commutator could be replaced by an in-built three-phase ‘bridge-rectifier’ stack – which coincidentally also did away with the need to have a ‘cut-out’.
Consequently, the Alternator could also be designed to have a larger frame diameter/number of field poles, resulting in it’s ability to produce not only much higher but sufficient output even at idling speeds to keep charging the battery, even with the head lamps on!

Soon, the Electro-Mechanical external Voltage Regulators too gave way, by the mid ‘80s, to in-built solid-state regulators, making the complete package far more robust and reliable as compared to a Dynamo.

Q22: Why the four stroke engines do not require 2T oil unlike the two stroke engines?

A22: As far as lubrication is concerned, even four strokers do require oil. However, the oil is almost always stored in the ‘sump’ below the engine in case of four stroke engines. Consequently, it has a Crankshaft driven Oil pump, which ‘circulates’ the oil to the desired areas, along with supplementary ‘splash’ lubrications to the Cylinder walls in some designs.

Two stroke engines cannot utilise an oil sump/pressurised lubrication system as above, as the sump over here is used as a ‘compression’ chamber during part of the two stroke cycle. Therefore, to provide adequate cylinder wall lubrication, a small percentage of specially formulated/low combustion residue oil (2T) is mixed with petrol to do the needful.

Further, the inevitable use of 2T oil in 2stroke engines is not very environmentally friendly, as the oil also burns away along with Petrol. In addition, since there is no (forced) Exhaust Stroke in a 2-stroke engine,
one has to resort to ‘over scavenging’ of exhaust gases, resulting in some un-burn air-fuel mixture also getting thrown out.

Q23: Which has a longer life span – a two stroke or a four-stroke engine? Why?

A23: Theoretically and practically, a small 2-stroke engine has a lower lifespan, since there is a bang/power stroke in the cylinder once every two strokes, as opposed to the once every four strokes in the four-stroke engine. Consequently, they can and do operate at higher RPMs, yielding practically double the Power to weight Ratio as compared to a 4-S engine.

Content generated by S. K. Gupta. - 10/’02.