Fundamentals of Automobile Engineering Part 1

Apart from answering your questions on the ‘Ask an Expert‘ section of our website, we decided to do a Q&A covering the fundamentals of automobile engineering.

A surf around our site will reveal that during the coming times, we have all the intentions and resources of giving you at the click of a mouse, what most other automotive sites/magazines seem to lack.

In other words, culled out of our years of hands down experience, Clinical analysis’ and remedies of performance.

Since we are all only too well aware that today’s Cars are a far cry from the good old Ambys and Fiats of the 1970-80s and there is a lot more to come, it’d be worthwhile for most auto-enthusiasts to first get better acquainted with the basics of Car Design and what makes a car tick the way it’s intended to.

What follows, therefore, is an attempt to bring about a greater technical awareness amongst Motorists in a simple QnA format spread over four ‘editions’ - as to what makes their Cars tick.

Q1: What is PS? What is its relation with BHP?
Answer:
‘PS’ stands for ‘Pferde Starke’ and it’s a unit of Power Measurement. It was popular in post-war Germany and still in use there. One PS is slightly less than one HP (1 HP ~ 1.07PS).

Q2: Why are four cylinder engines more stable, refined and smooth in operation compared to three-cylinder ones?
Answer:
Simply put, a Four Stroke Engine needs 2-revs to complete a 4-stroke cycle. Consequently, when one cyl is in its ‘compression/power sapping’ mode, ‘simultaneously’ there is another one undergoing its ‘power’ stroke. This results in a fair ‘power balance’ at the flywheel of the Engine.

On the other hand, in a 4-stroke/3-cyl Engine – during a period of 2-revs, only 3-cyls are firing. So there is a ‘fluctuating’ ¼ of a cycle period during which no cyls are firing (power stroke) and the other one/two are in their compression/power-sapping mode. This results in appreciable ‘Power Unbalance’ at the Flywheel, leading to unacceptable vibrations.

To overcome this inherent limitation, present day 3-cyl engines employ various techniques to smoothen out such vibrations. It may be interesting to note here that a 5-cyl engine doesn’t suffer from such an inherent limitation and was used first by Audi in the ‘60s with great success.

Q3: Why ‘Multivalve’ Engines are more fuel-efficient than ‘Two-Valves/Cyl’ ones?
Answer:
Given the basic Bore/Stroke and Compression Ratio of an ICE, its Power Output is directly proportional to the ‘weight’ of Air/Oxygen that it can draw-in to burn the matching Fuel quantity and inversely to the effort it has to make to expel the Exhaust Gases. Multi-Valve engines therefore have better ‘Volumetric’ efficiency/Specific Power Output than one-in and one-out types.

Q4: Why the four stroke engines do not require 2T oil unlike the two stroke engines?
Answer:
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 under pressure 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.

Q5: Which has a longer life span – a two stroke or a four-stroke engine? Why?
Answer:
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 – thus resulting in comparatively shorter life.

Also read:
Fundamentals of Automobile Engineering Part 2
Fundamentals of Automobile Engineering Part 3
Fundamentals of Automobile Engineering Part 4