Fundamentals of Automobile Engineering Part 4

With more and more new generation cars hitting the market as a result of the on going economic liberalization, car buyers are being bombarded with a slew of automotive jargon that leaves most of them quite confused.

In this concluding Part 4, we wrap-up some of the remaining FAQs that most motorists look for answers and trust that the surfing through parts 1 to 4 would leave them somewhat wiser than before.

Q14: How does a Variable Power Assisted Steering System work?
Answer:
The main objective is to achieve as uniformly and reasonably effortless a ‘feel’ of the Steering throughout the various ‘steering angle’ conditions and Car speed. In other words, it calls for ‘max assist’ during parking maneuvers and practically zero during straight cruise conditions.

In a Hydraulic System, this is achieved by progressive transmission of higher and higher PS Pump pressure to the actuating mechanism, depending inversely on the engine rpm and directly on the degree of steering wheel turn. The unwanted pressurised fluid returns to the Hydraulic Pump via a by-pass/relief valve.

In the case of an All Electric System, the on board ECU decides the degree of Assist, based on the ‘stress induced’ on the driver side of the Steering Wheel Shaft, by altering the Voltage Supply to the ‘Assist Motor’.

Q15: What is ‘Torque Steer’ and what are its disadvantages?
Answer:
This is a phenomenon peculiar to FWD Cars – the more powerful –the worse ! What actually happens is:

Given the practical Power Train layout in FWD Cars, the Differential ends up being off-centre relative to the axis of the Car.

This in turn results in one of the drive shafts (usually the lhs) being shorter than the other – as we are all aware of.

Since the ‘Torsional Stiffness’ of the longer DS is lower than the shorter one – both being capable of transmitting the same power – under sudden Surge of High Power such as during a hard acceleration, there is a fraction of a second ‘delay’ in the 100% Torque appearing at the Wheel end of the Longer DS than the shorter one.

As can be visualized therefore, such a ‘delay’ results in the Car ‘noticeably’ pulling to the longer DS side, briefly, when accelerated very hard. In view of this, High Powered FWD Cars are now being designed with their Diff so positioned that it results in equal DS lengths on both sides.

Besides this, torque-steer can appear even in conventional/low powered cars if the ‘road holding’ of either side drive wheel is different than the other – for whatever reason.

Q16: A Tachometer consists of a red zone at high rpm markings. What happens actually after that limit?
Answer:
Upto the beginning of the ‘Red Line’, an ICE can rev safely/repeatedly without any internal damage. If revved beyond/into the Red Zone, it stands to get damaged due to inadequate Lubrication/Cooling and therefore, Mechanical/Electrical ‘Speed Governors’ are provided in the engine management system to prevent such a mishap.

Q17: What is coefficient of drag? How can it be known? What is its significance and what is its unit?
Answer:
A Car has a ‘profile’ – endeavoured to be practically as ‘aero-dynamic’ as an Arrow but at the same time it also has a square or a rectangular cross-section’ related to its W x H.

If such a ‘cross-section/flat board’ is ‘inflicted’ with a blast of air in a Wind Tunnel corresponding to the road speed of the Car, it would experience a ‘push’ of say, 100 kgs.

On the other hand, if the actual Car is put in the place of the ‘flat board’, it would experience a push much lower than as above, coz of its ‘aero-dynamics’ – say 30 kgs.

Therefore, the ‘Coefficient of Drag’ is the ‘ratio’ of the above two i.e. 0.30.

Since it’s a ‘ratio’, it has no Unit and obviously, the lower it is - the better. Present day ‘Stock Cars’ have managed to achieve a “cee-dee-alpha” (coefficient of drag) of less than 0.30 – typically 0.25.

Q18: What is Engine Life Factor (ELF)? How can it be calculated?
Answer:
It’s a ‘Factor’ given by the Formula ‘ELF’ = 100,000/Max RPM x Compression Ratio’ of an ICE. In other words, higher the operating Rpm and/or the compression ratio, lower the ‘ELF’. Since it’s a ‘number’ only, it’s devised to ‘compare’ the Life and Reliability amongst comparable ICEs.

Q19: What is Terminal Engine Meltdown?
Answer:
It’s the ‘irreparable’ damage to the internal moving parts of an ICE, caused by overheating, either due its Lube OR Cooling System failure.

Also read:
Fundamentals of Automobile Engineering Part 1
Fundamentals of Automobile Engineering Part 2
Fundamentals of Automobile Engineering Part 3