Not sure that I entirely agree with HJ?s comment in the DT today, in response to a letter from DS HJ says that one should not rev an engine before switching off because it will leave the cylinders flooded with fuel. However with regard to a modern fuel injected normally aspirated petrol engine the fuel is cut off as soon as the ignition is cut so a few extra revolutions of the engine will leave it with fresh air in the cylinders as opposed to fuel or acidic exhaust gasses.
Clearly this is not the case with a carburetted engine, or a turbocharged engine where it should be left to idle so the turbine slows down prior to losing oil pressure.
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But why would you want to rev an engine before switching off?
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theres more chance of unburnt fuel being in the cylinders if you rev it and turn off as not all the fuel would be burnt straight away,look at a co analyser next time you go for an mot and boot the throttle the co goes barmy for the first few seconds.
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I always thought it was something idiots did with expensive or "modded" cars.
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On any turbocharged engine, don't do this! It can leave the turbo spinning at very high speed for a short time after the engine stops, starving its bearing of oil. If done habitually, the turbo will fail prematurely.
Cheers, SS
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Doesn´t HJ recommend ´simmering´the turbo though, by idling the engine before swithcing off, particularly after a long run? Stops the oil wrecking the bearings or something.
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Doesn´t HJ recommend ´simmering´the turbo though, by idling the engine before swithcing off, particularly after a long run? Stops the oil wrecking the bearings or something.
Simmering is good, but as has been pointed out on numerous occassions. If you've come off a high speed run from a motorway by the time you've got to a parking bay in a service station then that is usually good enough to get rid of any heat and got the turbo back down to a reasnoble speed....BUT remember this just stopping dead a engine that has been working hard the tempreature actualy increases for a few minuets as there is no cooling effect from the engine circulating the coolants, This is what has been said before about cooking turbo bearings and other numerous engine thermal failures. Turbo or non turbo be kind to your engine.... Which is warm up gently under load & cool down gently until no load... All this too means the minimal of idling
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Not sure that I entirely agree with HJ?s comment in the DT today, in response to a letter from DS HJ says that one should not rev an engine before switching off because it will leave the cylinders flooded with fuel. However with regard to a modern fuel injected normally aspirated petrol engine the fuel is cut off as soon as the ignition is cut so a few extra revolutions of the engine will leave it with fresh air in the cylinders as opposed to fuel or acidic exhaust gasses. Clearly this is not the case with a carburetted engine, or a turbocharged engine where it should be left to idle so the turbine slows down prior to losing oil pressure.
Yes all cars now have electronicaly controled fueling systems so please explain why on earth one should want to rev the nuts off any engine which is not on load?
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My Discovery manual advises to leave the engine idling for a few seconds.
No revving and the turbo last longer.
H
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Fit one of these to prevent turbo wear: www.accusump.com/acc_products/acc_turboiler.htm
It releases oil into the turbo after swith-off as it spools down.
Much larger versions are available for trucks and boats.
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The point is, yes, turbocharged engines should be left to idle so the turbine slows down prior to losing oil pressure irrespective of temperature.
However with regard to a normally aspirated fuel injected petrol engine the fuel is cut off as soon as the ignition is cut therefore a blip of the throttle will allow a few extra revolutions of the engine after the fuel is cut so will leave it with fresh air in the cylinders as opposed to a either fuel or acidic exhaust gasses.
This is not the case with a carburetted engine.
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A bigger factor than this is oil quality. If you have an EVO or similar then nothing but Mobil 1, (or similar fully synthetic if you must), should go near it. If you have an "ordinary" engine at least use a good semi-synthetic oil. I've a TDI and use only Millers XFE diesel specific semi synthetic, (or the Millers fully synthetic Diesel specific when I can't get XFE).
engine is now over 180,000 original turbo, never been apart, and uses no oil between changes. I had an audi 100 (5 cyl petrol) back when the aero shaped one first came out (1983) that car did 336,000 miles on original engine, never came apart, and I changed plugs & oil every 20K only ever using Mobil 1. It only died when an escort pulled out on it, (made a real mess of the Escort), audi was reparable but due to the age / mileage uneconomical .
One last important point the only way to ensure a good quality oil is going in to your engine is to put it in yourself, or stand over the person and watch them put it in from a sealed can you supplied. You'd be horrified how many cars get feed "standard oil" and the mechanic uses your expensive oil in his or sells it to his mates. If you are at all suspicious get an oil analysis done, it can also give a good indication of engine condition and life left...
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Most modern engines will cut the fuel supply if the engine is "coasting" above a certain rpm. Therefore I imagine that if you blip the throttle to say 4000rpm, let it go, and momentarily switch off the ignition before it's had a chance to return to a fuelling rpm, then you'll end up with lovely fresh air in the cylinders.
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Mike Farrow
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A turbocharger takes about half a second to spin down, it's not a helicopter!
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Can't take that short a time. Its spinning in oil, very light weight at several tens of thousand rpm. Must take at least 30 seconds to come to a total stop from maximum revs. The spinner in my health club for drying swimming trunks spins for several seconds after you switch off.
Do you mean half a minute?
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Espada III - well if you have a family and need a Lamborghini, what else do you drive?
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The impeller in a turbocharger is spinning in air. Air which enters is 'stationary' and at a rate of about 0.04-0.18 kg/s.
The impeller has a low mass, and the drag of the bearing and the stationary air soon slows it down. If you don't believe me, take the air outlet pipe off and watch. It's not a vacuum cleaner with a 5Kg motor keeping it going. This is true for small turbochargers such as fitted to cars, but not to ships. If a turbo took 30 secs to spool down, it would have ONE HELLISHLY LAGGY like 20 seconds.
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It's also the heat energy in the exhaust gases that spins the turbo more than the airflow through the exhaust, the other end is a turbine that is trying to create above atmospheric pressure, (boost pressure), which is a very good air brake so it will slow down pretty quickly when you stop driving the exhaust end.
Again the key is oil. With a good quality oil I'd say you could rev it as much as you like and turn off when it's not under load and I doubt you'll damage it as it wont actually spin up very fast. Most turbo & engine damage can be attributed to oil quality, and lack of regular change intervals.
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Can't take that short a time. Its spinning in oil, very light weight at several tens of thousand rpm. Must take at least 30 seconds to come to a total stop from maximum revs. The spinner in my health club for drying swimming trunks spins for several seconds after you switch off.
The drier has a fan and motor attached which act like a flywheel with one air brake, the turbo has two fans attached which act like two air brakes.
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The practice of blipping the throttle before switching off goes back to the days of temperamental carburettors and manual chokes. It was thought by some people that the unburnt petrol left in the cylinders would aid starting the next time. To my knowledge it was declared to be detrimental to the engine as much as 50 years ago. And I never knew anyone that could prove it made starting easier. By and large I think it was an old wives' tale.
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L\'escargot.
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You'd never think it to look at me, but I am actually old enough to remember being told to blip the throttle before turning off.
I believe that it was something to do with putting an extra bit of charge into the battery, so that you stood a half decent chance of starting again next morning.
It's only just now as I come to write it down, that I realise what carp advise that was!
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In order to ensure that it would start the next morning I was taught to stop, put the hand brake on, then switch off all anciliaries, then kill the engine ( which effectively gives in 5 seconds of idling).
In the morning start engine, then set lights, wipers, air con, then drive off.
It drives me bonkers when I get in the wife's car, put the key and on comes the radio, aircon and wipers.
MPZ
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