Some basics of Turbo-charging

Preamble
Nowadays, almost all BS-II/III diesel vehicles are fitted with ‘Turbo-Chargers’. These come in different configurations, depending upon the design philosophy of the vehicle manufacturers. So let’s see what’s this Turbo-Charging all about……

Some Basics

1) In Turbo Charging, the exhaust energy of the flow-out gases is used to drive a small Turbine, to which is directly coupled a small Turbine type compressor. The output of this compressor is linked to the ‘Inlet Manifold’ of the Engine.

2) Such TC’s can easily be made to spin at speeds > 100,000 rpm - which enables their size to be compacted because for a given output, the device diameter is inversely proportional to its rpm.

3) When air is so compressed - especially in a TC where the adjoining Turbine section runs @ +500*C or so - being the exhaust gases temp. - the air delivered by it too comes out much hotter - say > 60*C.

4) Since an Internal Combustion Engine actually looks for the suitable ‘weight’ of Oxygen required to burn a matching weight of fuel, rather than temperature dependent volume, it’s desirable to limit its inlet air temps to ~ 30*C or so - for it to deliver its best thermal efficiency.

5) Therefore, to lower the TC delivered air Temp from ~ 60*C+ to ~ 30*C, it has to be cooled by passing it through a suitable ‘Radiator’ - just like the Engine Coolant System. Such an ‘Air-Cooling Radiator’ is called an ‘Inter-Cooler’, coz it’s an ‘intermediary’ between a TC and the Engine.

6) A well designed Turbo Charger/Inter-Cooler System can enable a Naturally Aspirated Engine of the same ‘cc’ deliver easily upto +30% Additional Power - PROVIDED the internals of it are suitably designed also, to handle such levels of power delivery. DIY enthusiasts trying to TC an existing/NA diesel would do well to keep the following in mind :

i) Get its ’static’ compression ratio down to 16-17:1 a/a normal 0f 22+:1.

ii) Also ensure that its vital internals like Crank/Bearings/Conrods/Clutch etc can withstand the additional 30% or so power over its ‘NA’ rating that TC will bring about.

All this/aforesaid is easier said than done, at least as a retrofit.

Some relevant QnA’s

Q1: I was wanting to know when does the turbo come on in the Verna, I mean at what rpm…they say its variable geometry turbo but I’m unsure when it comes on.
A1: Being a ‘VGT’, theoretically speaking the TC ‘cuts-in’ from idling speed upwards itself, like an ‘Alternator’ vis-à-vis ‘Dynamo’s of yore, but obviously not in full strength. The degree of boost keeps building up progressively upto the designed limit - usually + 5 psi over atmosphere. Most TC’s reach this level around 1750 rpm.

Q2: As per OHB instructions, when you stop the vehicle, they recommend a 1-minute ‘cooling off’ time for the TC and not sure whether in routine city use the TC comes in!!!
A2: Your understanding of it is not right. The 1-min wait is recommended for the TC to rev down ’safely’ from its normal 50,000 rpm+ to about 5 k-rpm or so at idling and not to cool down, without losing the pressurised engine oil lubrication system of which it’s an integral part. If one were to switch off the engine immediately after coming to a stop for good, as we do for petrol’s w/o TC’s, then there’s a good possibility that the TC bearings may seize. Likewise at start-up/take off, as is recommended for the rest of the engine, for oil circulation to set in fully incl the TC, as it stands to rev up to 50k rpm from ~ 5k rpm in no time.

Q3: Only if I rev hard do I feel a jolt sort of acceleration at times so how am I sure that when the car is going to halt and I kill the engine whether the TC was in use!!! Is there a way of finding it out.
A3: I think this is now covered in the above 2-explanations ?

Q4: I don’t know if i should worry so much on this!
A4: Just pause for ~30-60 secs after firing the engine first thing in the morning before taking off, and that too gently as always. Likewise when you’ve to switch off the engine for good. Diesels all over the world are kept on idle most times, incl traffic lights/jams, may be for this reason alone - not to mention sub-zero temps that turn diesel fuel into a jelly of sorts. For such s/z duties, diesel fuel tanks of heavy commercial vehicles have electric heaters in their fuel tanks, which have to be switched off before refuelling.