Fundamentals of Automobile Engineering Part 2

Most of us are only too well aware that a lot has changed in the Country’s Auto Scene during the last 20+ years since the ubiquitous Maruti-800 first hit the roads – atleast by way of how present day cars are made and marketed. And with the economic liberalization on in full swing, there’s a lot more to come.

So here in Part 2, we continue with our attempts to bring about a greater technical awareness amongst Motorists as to what makes their Cars tick.

Q6: What is the ‘Compression Ratio’ of an Engine and how does a manufacturer fix it when they design an engine?
Answer:
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 :

i) The ‘CC’ of an ICE is the max volume of Water its Cylinder(s) 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 cylinder volume of an M800 is 796/3 = 265 cc.

ii) The ‘Compression Ratio’ (CR) of an ICE is defined as “Swept Volume + Clearance Volume/Clearance Volume”. From (i) above, it may be inferred that CC = SV+CV.

iii) Every ICE is designed and produced to have a CR as one of its 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 91-93. An M800 is, therefore, designed for 87 Octane with a CR of 8.7:1.

iv) 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.

Q7: What are Gear Ratios?
Answer:
Due to the inherent ‘Torque vs Rpm’ Characteristic of an ICE i.e. with its Torque rising practically from nil at idling to the max somewhere midway in the rpm range, one needs ‘suitable gearing’ to ‘match’ the road speed/acceleration related Torque/Power requirements of the Car to the Engine’s Torque vs Rpm characteristics, to enable the Torque required by the Wheels match the one the engine can develop.

Since the Wheels’ Torque requirement varies from take-off to cruising, one needs a ‘variable’ Gear Ratio to make the ‘transition’ as smooth as possible.

Hence in practice, five forward (and of course one reverse gear) of ‘appropriate’ ratios are provided – starting from, say, I/Reverse - 3.5:1, II - 2:1, III - 1.5:1, IV - 0.9:1 and V - 0.8:1.

On top of these, there is the fixed ‘Final drive/Differential’ Ratio of say 4.5:1, which stands to be ‘multiplied’ to all the five/six above, to give the ‘Overall Wheels to Engine’ Gear Ratio, in any given gear position. The ‘Ratio’ of any two mating gears is the Ratio of their respective number of teeth.

Q8: Why Automatic Transmissions are more thirsty for fuel than their Manual counterparts?
Answer:
An AT uses a ‘Fluid Coupling’ instead of the conventional mechanical clutch, to eliminate the need for its external manipulation, in order to make it fully ‘Automatic’. A Fluid Coupling basically comprises a pair of Turbine-like ‘Rotors’, one of which is coupled to the Engine and when ‘driven’ by it, develops pressure in the surrounding ‘Fluid’. This in turn ‘tends’ to ‘drive’ the other ‘Follower’ Turbine Rotor, which is coupled to the AT. As can be visualized, such a Fluid Coupling will always have some ‘slip’ even when ‘fully coupled’ and this inevitably results in constant ‘churning’ of the ATF resulting in some Power loss – leading to higher fuel consumption – typically 5-10%.

However, with the advent of ECU controlled MPFI Engines, the ‘commands’ to the AT are now given by the ECU, which make sure that the Car is always in the ‘right gear’, under all possible driving conditions.

This in turn results in overcoming the lack of Driver Skills towards timely Gear Changes and therefore, today’s ATs are almost as Fuel Efficient as their MT counerparts.

Q9: What makes a ‘Distributor-less’ Ignition System better than the conventional ones and how does it work?
Answer:
In a conventional Ignition System, the ‘Primary’ Circuit of the ‘Ignition Coil’ has to be continuously ‘interrupted’ by ‘electro-mechanical’ means, such as the Distributor ‘CB Points’ (Pre-MPFI M800’s) or the semi-electronic variants of it (Zen/Esteem).

In a DB-Less System, the on board ECU performs the ‘primary coil interruption’ function electronically in a ‘contactless’ manner. Since it does away completely with all moving parts, it’s considered to be more stable and reliable and ‘theoretically’, having an infinite life.

Also read:
Fundamentals of Automobile Engineering Part 1
Fundamentals of Automobile Engineering Part 3
Fundamentals of Automobile Engineering Part 4