Entries from September 2006 ↓

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

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.

So here in Part-3, we continue with our attempts to bring about a greater technical awareness amongst Motorists as to exactly does the new marketing jargon mean to them.

Q10: What is the difference between an Alternator and a Dynamo as far as working, efficiency and performance is considered? Which is better and why?
Answer:
Any Rotating Electrical Machine is inherently a ‘multi-phase’ AC Device and NOT DC !

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 of yore and even in present day 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 having 3x the power output capability compared to a Dynamo – which coincidentally also did away with the need to have an external Relay type ‘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 and other loads on!

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

Q11: Why do Diesel Engines feel more sluggish with the AC ‘on’ than their petrol cousins?
Answer:
The ‘Size/Power requirement’ of an Car A/C System is dictated by the Cabin volume AND the initial temperature (can get as high as 70*C for a Car parked in the Sun) and the rate of initial cooling desired – amongst other factors which are common to most A/C Systems. This results in an average Car AC System to have a Rating of almost 1.5 Tons 0r 3 Bhp when on!

A diesel engine has a lower ‘specific power’ output compared to equivalent petrol – typical Example Accent-D (57 Bhp) a/a Accent-P (94 Bhp). In the mid driving range it’s ~ 50% of that. Therefore, with a more or less ‘constant’ A/C load of 3 Bhp, the ‘drag’ works out to a much higher % age of Power available in a Diesel a/a Petrol.

Q12: What is a ‘Common Rail’ Diesel Engine? How does it work?
Answer:
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 pressure of about 1500 bars, as against only 2-3 bars of petrol Mpfi and 400 bars of conventional diesel engines !

In Crdi’s, 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 ECU commands, and ‘squirt’ in multiple steps 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 Fops hitherto in use.

Q13: How many Power Steering Systems are used presently? Which is better in all aspects, such as performance, reliability, economy, etc.? How do they work? Which types corrupt the vital engine horses more and Why?
Answer:
There are three basic power steering systems in use today – i) Hydraulic, ii) Electro-Hydraulic and iii) All Electric.

As they say, there are no free lunches in this world. All the three eventually derive their power input requirements ultimately from the Engine, either being directly powered off its Crankshaft OR the Car Battery/Alternator.

Of the three, The Hydraulic one is the most time-tested and
Popular, as it can be applied from a Small Car to a Giant Earth Mover. However, by virtue of the nature of its design, it’s more maintenance prone and a little less energy-efficient.

On the other hand, an All Electric ones presently have their application limited to Passenger Cars – weighing, say, from 750 kgs to 1500 kgs or so. This is coz it primarily depends on the Car’s Battery to Power it, which in turn depends on the Engine driven Alternator to charge it back. Since it can be ‘Computer Assisted’, it can very easily be programmed to any desired ‘spectrum’ of ‘Assist’. Since there are fewer moving parts in it and can be virtually made ‘idle’ at straight cruise conditions, it’s more ‘direct’ and Energy Efficient. An Electro-Hydraulic System is a ‘cross’ between the two, by way of advantages and disadvantages and doesn’t seem to be much popular these days, presumably from initial cost considerations.

Also read:
Fundamentals of Automobile Engineering Part 1
Fundamentals of Automobile Engineering Part 2
Fundamentals of Automobile Engineering Part 4

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

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