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