Entries from April 2008 ↓

Preamble
Ever since the first single-cylinder petrol engine driven Car appeared on the roads during late 19th century, a ‘Carburettor’ became synonymous with it. Later on with the advent of multi-cylinder Engines, so did the ‘Distributor’ or ‘Delco’s.

During the last Century, both evolved considerably – till the ‘Euro-II’ like Pollution Norms got the better of them. For, the Carbs worked on a fundamental principle of ‘reaction’, which could easily get stifled for many reasons. Whereas man, wanted to be literally in the Driver’s Seat – as always.

And thus, as the cliché of necessity being the mother of invention goes, a System of ‘controlled-feeding’ of Fuel + Air + Ignition Spark to the ICE Cylinders in the early ‘80s evolved. A ‘system’ like that had to be based on the engine’s ‘needs of the moment’, such as the load on it, its rpm, vehicle’s road speed etc.

Such a ‘system’ had to ensure that the ‘ob-Nox-ious’ Pollutants were kept to a bare minimum all over it’s operating range – not to mention squeezing the maximum ‘mileage’ out of the last drop of fuel that went into it. Thus ‘fuel-injection’ systems were born – replacing nearly a century old Carburettors.

For the uninitiated, ‘MPFi’ stands for ‘Multi point fuel injection’. Such a system ‘injects’ fuel into individual cylinders, based on commands from the ‘on board engine management system’ Computer – popularly known as the Engine Control Unit or the ECU.

MPFi Systems can either be: a) ‘Sequential’ i.e. direct injection into individual cylinders against their suction strokes, or b) ‘Simultaneous’ i.e. together or all the four or whatever the number of cylinders, or c) ‘Group’ i.e. into Cylinder-Pairs.

These techniques result not only in better ‘power balance’ amongst the cylinders but also in higher output from each one of them, along with faster throttle response.

Of these variants of MPFi, ‘Sequential’ is the best from the above considerations of power balance/output.

‘SEFi’’, as advertised by Ford, stands for ‘Sequential Electronic Fuel Injection’, which technically is the best of the above variants of ‘MPFi’. Hyundai/Maruti ‘MPFi’ systems are in fact ‘SEFi’ too. The erstwhile Cielo’s and Matiz’s had the (b) or (c) variants of above MPFi systems.

On the other hand, older Opel-Astra’s had a ’single point’ fuel injection system, which is in between an Mpfi and the now obsolete Single-Carburettor systems.

The ‘Fuel Injectors’ are precision built ‘Solenoid Valves’, something like Washing Machine Water inlet Valves. These have either single or multiple ‘Orifices’ which ‘spray’ fuel into the Fuel inlet manifold of a Cylinder upon actuation, from a common Rail/Header pressurised to around 3 bar, fed by a high pressure electrically driven fuel pump inside the Petrol tank of the Car.

The ‘on-board’ ECU primarily controls the Ignition Timing and quantity of fuel to be injected. The latter is achieved by means of controlling the ‘duration’ for which the Injector solenoid valve coil is kept energized – popularly known as the ‘pulse-width’.

In general, an ECU in turn is controlled by the ‘data input’ from a set of ‘SENSORS’ located all over the Engine and its Auxiliaries. These detect the various ‘operating states’ of the Engine and the performance desired out of it. Such Sensors constantly monitor: 1. Ambient Air Temperature, 2. Engine/Coolant Temp., 3. Exhaust/manifold temp., 4. Exhaust ‘O2’ content, 5. Inlet manifold vacuum, 6. Throttle position, 7. Engine rpm, 8. Vehicle road speed, 9. Crankshaft position, 10. Camshaft position, etc.

Based on a ‘programmed’ interpretation of all this input data, the ECU gives the various ‘commands’ to the Engine’s fuel intake and spark ignition timing systems, to deliver an overall satisfactory performance of the Engine from start to shut down, including ‘emission control’.

This/Part-1 of a 4-part Article is an attempt to familiarise the average ‘BS-II/III’ Car Owner as to what actually lies beneath their Bonnets. In Part-2, we’ll talk about the do’s and don’ts relating to MPFi Cars. In Part-3, for the more curious ones, we’ll see how the stuff works and in Part-4, we’ll explore the commonly encountered problems and DIY-Trouble Shooting.

Immediately, to get the best out of an MPFi System, one should always use – a) The OE recommended Petrol Additives with the ‘Regular’ Unleaded or the new generation ‘Premium’ Petrol’s and b) NEVER Tamper with the OE Wiring Harness of the Car – EVEN to install the ubiquitous Music System OR any other Electrical Accessory - other than those ‘approved’ by the OEM/Dealer and ‘installed’ by him, as these are designed to suit the Car’s OE Central Wiring Harness ‘Couplers’ provided for the purpose. ‘BUTCHERING’ OF THE OE-CWH IS AN ABSOLUTE TABOO FOR MPFi CARS.

Preamble:
In the 12th March article, if not most, some of you may have read about the interesting facts behind AirCraft Tyres. We even invited Readers to answer at the end, as to what makes an ACT burst at take-off, as distinctly different than a more probable one during landing. However, there was hardly any response – barring one and that too not entirely correct.

So yours truly again e-picked the brains of his TyreGuru – Mr. Sudershan Gusain – and this is how he enlightened me…

Tyre burst or explosion, two different things tho’, is an interesting topic and internationally, very few studies have been made in this direction. Here are some general explanations about them and they apply equally to automotive as well as ACTs:

A tyre burst/explosion is not so common a phenomenon but:
• It is difficult to detect before it happens.
• It may have multiple causes.
• It can have tragic consequences.

Some figures from Canada indicate that tyre explosions account for 1% of all accidents leading to deaths over there.

Before we proceed further, we should distinguish a BURST from an EXPLOSION:

BURST
A Tyre Burst can be defined as due to ‘mechanical deterioration’ of tyre material, thus weakening its ‘structure’, resulting in a sudden and violent release of the air.

The causes of a tyre burst can be classified into 2 categories:

MECHANICAL CAUSES
• Excessive tyre inflation.
• Improper bead seating on the Rim.
• Structural fatigue of the casing.

CHEMICAL CAUSES
HEAT is the main cause that can lead to a burst of chemical origin. Various chemical reactions can happen when heat inside the tyre builds up. Thermal decomposition of rubber then occurs and it chemically breaks the tyre material into its original components viz oil, gas and char.

EXPLOSION
Explosion is a sudden expansion of a gas under pressure. Flammable gases with relatively low self-ignition temperatures can get formed inside a tyre due to heat and/or decomposition of tyre material due to many other causes. And when the internal temperature of a tyre becomes higher than the self-ignition temperature of such gases, an ‘explosion’ can occur. However, for it to happen, the following conditions are to be met:
• Concentration of enough inflammable gas.
• Inside tyre temperature higher than the self-ignition temperature of the gases formed within.
• Oxygen concentration inside the tyre higher than 5.5%.

A chemical explosion of such a nature is much more violent than a burst, as it occurs at much higher pressures - usually ~ 500psi or more.

For a tyre explosion to occur, a high degree of heat is required. The causes of such high heat generation are due to following possibilities:

The most common cause is ‘wheel welding’. Rubber will start to decompose around 250°C. With the increase in internal temperature, the mix of oxygen and flammable gas auto-ignites at around 450°C and an Explosion can occur.

Use of a welding torch to release bolt/nut.

Use of petroleum lubricant or solvent during tyre mounting.

Brakes’ overheat.

Electric discharge either from a high voltage line or lightning.

Fire in ambience.

Severe under inflation or overloading.

Presence of contaminants inside the tyre.

Oil or other combustible fluid ‘absorbed’ by the tire.

If we now go back to the original question i.e. Tyre burst or explosion during take-off, one can probably understand that we should first determine whether it is a burst or an explosion. They key causes in case of an aircraft would be:

These are also the reasons why ‘Nitrogen’ inflation as well as disc brakes is mandatory on Aircrafts.

Preamble
As is not unusual, the last week’s Article on the Life Cycle of an Automobile Engine evoked some pertinent questions from one of the regular e-readers of this column. I won’t be surprised if most of you had similar reactions. So this is how it went…

Question
Just went through your article on ‘Engine life’ and was curious about the line ‘Maximum dry CR 13.5 kg/cm @ 400 rpm’. I assume ‘dry’ means cranking the engine without allowing ignition to take place and ‘CR’ means compression ratio, but what does the figure 13.5 kg/cm stand for? Is it pressure (i.e. kg/sq. cm)? VG/Mumbai

Answer
‘Compression’ Tests are of 2-types - i) ‘Dry’ meaning as done below and - ii) ‘Wet’ meaning after injecting a 10-20 ml of engine oil thro’ the spark plug opening ‘before’ cranking the engine for the test. Ideally speaking, in a healthy engine, there shldn’t be much difference between the two. However, if the piston/rings are worn out, there will be significant difference. So a ‘wet’ test is a sort of double check before concluding that the p/r’s are gone.

It’s a std procedure when taking the ‘compression’ readings to - i) remove all the spark plugs/Air Filters, ii) depress the Accl Pedal fully and iii) crank the engine by the starter motor, which is usually 400 rpm per typical petrol engine starter motor design, till the pressure gauge screwed in place of a spark plug/cyl registers a max reading (2-fellows needed). The PG used is a ‘Stay-Put’ type, like a Tyre Pressure Gauge, which permits one to read it accurately and then ‘reset’ to zero.

The fig ‘13.5′ is - i) the ‘pressure’, either in psi or kg/cmsq here, recorded by the PG used as above and ii) ‘13.5′ is the fig for a healthy engine having a compression ratio of 9:1 - as per Suzuki Work Shop Manuals.

Question
Also, would the pressure not depend on the compression ratio, as well as the inlet pressure (in supercharged/turbocharged engines, this would be above atmospheric).

Answer
Obviously it’ll be higher for engines having ‘Compression Ratio’ higher than 9:1. At 400 rpm, atleast a Turbo Charger will have no effect coz the engine is prevented from firing by removal of its Spark Plugs and in a Super-Charged engine, the pressure build-up will be negligible at such a low rpm. May be they recommend taking off the S-C drive belt in such a case but I won’t know off hand as I’ve yet to come across a work shop manual an S-C car!

Question
Wouldn’t it also not depend on the condition of the air filter and the inlet/exhaust valves (more the fouling, less the pressure)?

Answer
As I’ve said above, the air filter is removed for such a test. I suppose valve/seat wear goes hand in hand with piston/rings wear, as far as an engine performance is concerned.

Subsequent Responses
Thanks for the very precise answers to my queries! It has improved my understanding of automobile engines. I remember I had the chance to witness a ‘dry’ test, after the garage had done a ‘carbon clean’ chemical process on my Maruti-800 a few years back.

However, instead of the ’stay-put type’ pressure gauge you mentioned, they had an instrument that took the compression readings on ‘cards’, much like an engine indicator (used in marine engines). He claimed that the improvement in readings was due to the ‘carbon cleaning’ of the valves/valve seats and pistons/rings. He even inserted a sort of probe inside the spark plug hole, to show me on a video screen the cleaned insides - an instrument similar to what ENT Doctors use to look inside one’s ears/throat! VG.

That was an interesting experience you shared with me VG. However, this ‘Carbon-Clean’ using machines for the purpose can be dicey and do more harm than good, as certain minimum carbon deposits at ‘strategic’ locations within an engine are actually necessary, to enable it achieve its highest possible/designed compression ratio. SKG.