jenevers Posted March 19, 2020 Report Share Posted March 19, 2020 What’s behind the theory of revving up your engine just before switching off. Is it not better to idle for a couple of minutes to let the engine cool down a bit? Link to comment Share on other sites More sharing options...
Sir Nibble Posted March 19, 2020 Report Share Posted March 19, 2020 It's just something people do for no good reason. I have heard it's so that "all the pistons are at the top ready for the next start"? Very bad for turbos. The Bristol engines fitted to WW2 bombers were run down gently by ground crew after landing to cool them because if they were shut down right away they were damaged by heat soak. Link to comment Share on other sites More sharing options...
Tony Brooks Posted March 19, 2020 Report Share Posted March 19, 2020 26 minutes ago, jenevers said: What’s behind the theory of revving up your engine just before switching off. Is it not better to idle for a couple of minutes to let the engine cool down a bit? I am with Sir N NEVER ever rev a turbo engine up before shutting down and tray to drive coats the last few yards at low revs, in fact some manuals tell you how long to let it idle for. I don't see it makes any difference at all for a non turbo engine apart from wasting a little fuel. Link to comment Share on other sites More sharing options...
alan_fincher Posted March 19, 2020 Report Share Posted March 19, 2020 7 minutes ago, Sir Nibble said: I have heard it's so that "all the pistons are at the top ready for the next start"? could be true if it was a Bolinder! ? Link to comment Share on other sites More sharing options...
RLWP Posted March 19, 2020 Report Share Posted March 19, 2020 1 minute ago, alan_fincher said: could be true if it was a Bolinder! ? Or a two cylinder Kingfisher Richard Link to comment Share on other sites More sharing options...
BWM Posted March 19, 2020 Report Share Posted March 19, 2020 I think some just like to finish on a flourish, there is no mechanical advantage. Link to comment Share on other sites More sharing options...
jenevers Posted March 19, 2020 Author Report Share Posted March 19, 2020 I always idle for a few minutes before shutdown. But there must have been some theory for the revving up in days gone by? Link to comment Share on other sites More sharing options...
RLWP Posted March 19, 2020 Report Share Posted March 19, 2020 4 minutes ago, jenevers said: I always idle for a few minutes before shutdown. But there must have been some theory for the revving up in days gone by? I suspect it was to do with 'cleaning the plugs' on high performance petrol engines. A final burst to get the electrode hot and burn off any oil from idling making the next start easier Doesn't make any sense on a diesel Richard Link to comment Share on other sites More sharing options...
NB Esk Posted March 19, 2020 Report Share Posted March 19, 2020 (edited) Revving an engine just prior to switching off is a throwback to the "old" days of motoring. The theory being a quick blip of the throttle ensured the float chamber was as full of petrol as possible, thereby ensuring better restarting. It doesn't apply to modern engines and especially not turbocharged engines. The last thing needed is to set the turbo spinning just prior to starving its bearings of oil. Hope this helps..... Edited March 19, 2020 by NB Esk Autocorrect 3 Link to comment Share on other sites More sharing options...
Athy Posted March 19, 2020 Report Share Posted March 19, 2020 17 minutes ago, BWM said: I think some just like to finish on a flourish.. I blame Chuck Berry, who "gunned the motor twice" for that very reason in 'No Money Down'. Not sure he's the best role model, mind. Link to comment Share on other sites More sharing options...
RLWP Posted March 19, 2020 Report Share Posted March 19, 2020 2 minutes ago, Athy said: I blame Chuck Berry, who "gunned the motor twice" for that very reason in 'No Money Down'. Not sure he's the best role model, mind. I don't think I would take motoring advice from him, his work ethic was well worth following Richard Link to comment Share on other sites More sharing options...
Sir Nibble Posted March 19, 2020 Report Share Posted March 19, 2020 1 hour ago, alan_fincher said: could be true if it was a Bolinder! ? Only if you stop it with a decompressor. Link to comment Share on other sites More sharing options...
OptedOut Posted March 19, 2020 Report Share Posted March 19, 2020 It is not good practice to shut an engine down immediately after a period of 'loaded running', the last thing an engine needs is a good revving prior to the immediate shutting off of the cooling system. Link to comment Share on other sites More sharing options...
bizzard Posted March 19, 2020 Report Share Posted March 19, 2020 When switching off there is a short term climb in cylinder head temperature as the coolant circulation stops. Proof of this is sometimes after you've switched off and walking away the cooling fan switches on to check it. 1 Link to comment Share on other sites More sharing options...
roland elsdon Posted March 19, 2020 Report Share Posted March 19, 2020 Turbo diesels in hot climates are often fitted with a turbo timer that shuts the engine down after you have wandered off and locked up. (To protect the turbo bearings. ) At 40 degree ambient the engine can run for some time. Link to comment Share on other sites More sharing options...
RLWP Posted March 19, 2020 Report Share Posted March 19, 2020 42 minutes ago, bizzard said: When switching off there is a short term climb in cylinder head temperature as the coolant circulation stops. Proof of this is sometimes after you've switched off and walking away the cooling fan switches on to check it. Or boil over Link to comment Share on other sites More sharing options...
Tony Brooks Posted March 19, 2020 Report Share Posted March 19, 2020 45 minutes ago, bizzard said: When switching off there is a short term climb in cylinder head temperature as the coolant circulation stops. Proof of this is sometimes after you've switched off and walking away the cooling fan switches on to check it. and some have small electric cooling pumps to keep the coolant circulating. Link to comment Share on other sites More sharing options...
zenataomm Posted March 19, 2020 Report Share Posted March 19, 2020 After World War Twice my dad left The RAF and drove buses for a while. Western National instructed their drivers to ..... - Leave their engines on tick over when stopping for breaks or at terminus turn rounds as it used less fuel than starting them up again and hammered the batteries less. - Rev the engine before shut down to blow out all the clag and soot from around the exhaust valves. To avoid sticking valves and poor compression on start up. Link to comment Share on other sites More sharing options...
cuthound Posted March 19, 2020 Report Share Posted March 19, 2020 As already said by others, no point in revving any engine or letting the engine "cool down" prior to shutdown for normally aspirated engnes. When commissioning turbocharged diesel engines, it was usual to run them at 100% load for 11 even hours, followed by the 12th hour at 110% load tp prove they met the specifcation. At the end of this test the turbochargers would be glowing red hot, and typically at 1200-1400°C. It was necessary to let the engine idle for 3-5 minutes until the temperature had dropped and stabilised. Failure to do this damaged the turbos as the hot turbo carbonised the oil left in it, which soon wrecked the turbo bearings. Link to comment Share on other sites More sharing options...
jenevers Posted March 22, 2020 Author Report Share Posted March 22, 2020 On 19/03/2020 at 18:55, cuthound said: As already said by others, no point in revving any engine or letting the engine "cool down" prior to shutdown for normally aspirated engnes Who said no point cooling down? Link to comment Share on other sites More sharing options...
LEO Posted March 22, 2020 Report Share Posted March 22, 2020 My Dad always blipped the throttle prior to turning off the igntion.....not sure why, he thought it 'primed' the cylinders prior to starting (1930's thinking). I always idle the engine I have as it has a turbo. The boat diesel just get the fuel shut lever pulled. Interesting about the Bristol engines - radial?, the other thing to remember before staring is to drain the bottom cylinder to avoid oil compression problems, my mate forgot this and caused some expensive problems on a Pembroke plane he had. Link to comment Share on other sites More sharing options...
bizzard Posted March 22, 2020 Report Share Posted March 22, 2020 The Bristol Pegasus aero engine was the first aero engine to develop 1hp per lb in weight, which was governed and met by the petrol companies increase in petrol octane rating. For a while in the 1930's a monoplane with a Pegusus engine held the alitude record of 56,000 ft or so so, around twice the height of Everest. Link to comment Share on other sites More sharing options...
NB Esk Posted March 22, 2020 Report Share Posted March 22, 2020 25 minutes ago, LEO said: My Dad always blipped the throttle prior to turning off the igntion.....not sure why, he thought it 'primed' the cylinders prior to starting (1930's thinking). I always idle the engine I have as it has a turbo. The boat diesel just get the fuel shut lever pulled. A lot of people used to do this without really knowing why. The last time I came across it was 20 yrs ago when I recovered a Rolls Royce that had been driven through the back of a garage. The old fella must have got mixed up with his routine, which should have been something like enter garage, knock it in neutral, rev up, switch off and brake. Anyway, whatever went wrong the front end of the roller was stuck through the end of the garage. Funny thing was there'd been some shelves up above the window with tins of paint on, a couple of which had burst open and given the Spirit of Ecstasy a new look. Got some really odd looks as I drove round the Huddersfield ring road with that on the truck. Link to comment Share on other sites More sharing options...
Jrtm Posted July 3, 2020 Report Share Posted July 3, 2020 There are reasons for doing it, depending on the engine. After a long day you normally alow to run for a few mins as all the heads block ect will be very warm the idea is to alow the water to cool these parts before shutting off. If direct water cooled it will cool quicker. The idea behind a rev up then shut off is to use any final fuel in the lines and pump and cylinder / cylinders normally if the engine is then going to be left for a few days+ A direct shut off depending on engine will always inject fuel into the cylinders so you rev then shut off to stop this and use it up. It also comes from very old engines where by you hold the fuel pump fully shut or fully open to kill the engine by cutting the fuel. Some pumps there shut off is after a pump some before. If after you ov get a quick rev then the fuel cuts out again the idea is to use the fuel up in the cylinder and it cant work its way into the oil when stopped. This might not seam much but on 2 stroke diesel you burn a little oil if fuel gets in this oil over time you can end up with more fuel in oil causing run ons, so you use your fuel up before killing. Link to comment Share on other sites More sharing options...
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