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| Get the best out of your Car (Part-XII) | ||
| Preface
In Part-XI of this 'series', I'd shared with you some thoughts and real
life experience about different kinds of problems one can run into while
maintaining one's Car. -----------------------------------------------------------------
Coming to a Car with a 'multi-cylinder' 4-Stroke engine, not that a 2-stroke would make any difference, let's look at the following fundamentals : 1) We all know that an ICE has a piston inside its cylinders 'connected' to the 'Crank-shaft' by means of connecting rods. This way, the reciprocating motion of the pistons is converted into a rotary one of the Crankshaft - just like the exposed ones of a Railway Steam Loco. 2) The top-end of a piston's stroke is known a 'top-dead-centre/TDC' and like wise the bottom/BDC. 3) A 4-S/ICE has to have a minimum of 1-Inlet Valve and 1-Exhaust Valve. These valves are opened/closed at the designed 'timing' by the 'Camshaft', which is driven by the Crankshaft at 1/2 its rpm by means of either a 'timing-chain' or a 'toothed rubber belt' - as in most Cars today. 4) Stroke 'one' of a 4-S/ICE is called the 'suction stroke'. This happens when the piston moves downwards from its TDC with the inlet valve open. This allows it to 'suck-in' appropriate mixture of Air and Fuel - either via a Carburettor OR by 'injection' as in Mpfi Cars today. When the piston reaches the BDC, the inlet valve closes. 5) On its return stroke from such a BDC, the piston begins to 'compress' the A/F mixture it drew-in in the previous stroke AND during this stroke, both its I/E valves are closed. This/'Stroke-2' is called the 'Compression stroke'. 6) Just before it reaches the TDC - a point calculated by design/experience - the Electrical Ignition System of the Car sends a high voltage pulse to its Spark Plug. The pulse is strong enough to enable it 'jump across' the SP's calculated/pre-set 'gap' inside the Cyl. 7) Such a spark ignites the explosive A/F mixture within the Cylinder that is already compressed 12 to 13 times the Atmospheric pressure by now and it goes off like a bomb inside it. Both the I/E valves are still closed. 8) Since such exploding gases are still 'contained' within the cylinder, the piston gets pushed back to its BDC with all their force after it reaches its TDC on the upward stroke. This/'Stroke-3' is called the 'Power Stroke'. 9) Just when the Piston so hits its BDC, only the Exhaust valve is opened via the Camshaft and on it's such upward stroke #4 - the 'Exhaust stroke' - the burnt/expended gases are 'driven out' of the cylinder via the tail pipe by the time the piston reaches its TDC. At this point, the Camshaft closes the exhaust valve and soon thereafter - opens the Inlet Valve for the suction stroke to commence as per (4) above and the entire ‘cycle’ repeats itself. 10) In present day designs, there’s some calculated ‘overlap’ between closing of the exhaust valve and opening of the inlet valve, to facilitate what’s known as ‘Exhaust Gas Re-circulation’. This is a complex theory and we’ll keep for discussion for a later day. 11)
In multi-Cylinder engines – ‘4-inline’ being most common
today – each cylinder is offset with its neighbour by 180*, such
that if, say Cylinder # 1 is on its suction stroke, its neighbour will
be on ‘power’ stroke and the next on exhaust and the last
one on compression stroke. 2) Turbo ‘Inter-Coolers’ The advent of EU or BS-III Pollution norms has evoked some curiosity amongst people as to what does an ‘Inter-Cooler’ stand for – now going even with the most mundane of all diesels – the Indica. So let me begin from the beginning : The ‘exhaust-driven’ Turbo-Charger gets ambient air to compress like any other engine - out of the air-cleaner - but during such compression to ~ +5-bar, due to the elementary laws of physics it, it gets heated up in the process to easily 30*C+ over ambient - hence the need for an I/C to ‘cool’ it before feeding it to the engine to improve its 'volumetric efficiency'. Such cooling is reqd coz what the engine needs to deliver max power is max 'weight' of oxygen ‘per unit volume’ supplied to it and that otherwise gets diluted with its high temps. The -I/C works exactly like the main engine radiator except for the fact that instead of coolant circulating thro' it, it's compressed air coming out 'out' of the turbo that it's designed to cool - as much as possible - 'before' feeding to the engine. Obviously, this cannot get below the ambient temp as otherwise, it'd call for some air-conditioning ! And cooling it below ambient is not reqd either coz engines are happiest with inlet air ~ 30*C. If much colder, they lose power and therefore need it to be warmed-up before getting into the engine - a feature most cars operating in cold climes have - a 'summer/winter' flip lever on the air-cleaner - incl the original/S-800 launched in India and continuing with the present Gypsy. Such levers, when flipped to 'winter', take in fresh air from top of the Exhaust Manifold instead of straight front/side in their 'summer' posn, so that it gets warmed-up by the E-M heat before entering the engine. As far as physical location of the I/C is concerned, all that's needed is flow of fresh/ambient air thro' it - especially when the vehicle is in motion. Therefore, each OEM has it's own design on this front - given their physical location constraints in the engine bay. The ‘air scoops’ seen on some turbo-charged cars are meant to direct ambient air to the I/C so positioned under it - tho' in the latest Scorpio the scoop is just ornamental ! |
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| Author: S K Gupta |
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