some random trivia, but not strictly off topic. engine wear during cold starts IS NOT because oil isnt distributed around the engine fully (as suggested by magnatec and other oil brands) but because the oil isnt at sufficient temperature for it to remove and suspend the caustic/corrosive/abbrassive elements introduced into it from combustion. even 5 quid supermarket mineral oil "sticks" to engine internals like castrol tells us magnatec does. on the subject of turbo spinning at idle, the majority of energy spinning the exhaust turbine is heat, not just gas blowing through it
Ta Dave! All helpful stuff. Has there been a thread on running a car for a really short journey. e.g. Before it gets up to temp, and just leaving it? Can that have any lasting effects? Just curious.
Wears the engine, because it's always running rich and washing the bores. Also mayo build-up if the engine never gets properly hot.
What? The turbine spins because gas blows through it, the gas blowing is due to heat in a roundabout way. Turbos do not just spin with heat, they spin by gases passing them. Try putting a turbo in an oven, it won't do a thing......
of course a turbo will stop spinning when the engine is off always good to let them sit and idle after a good run and allow oil to circulate round, in the VW handbook it suggests that for NA cars too. I think some folk have a real misconception on how a turbo works
at no point did i say or intend to imply that latent heat alone drives the turbine. which you are clearly saying that i did, from your conventional oven example. sorry if you are offended by your own assumption, but i do take exception to the "what ?" as if what i have written is of no technical merit. i said the majority of the energy driving the exhaust turbine is heat. not just gas blowing through the turbine. meaning if you had an imaginary starter motor that was able to crank an engine to 4000rpm, it would not be driving the exhaust turbine at a quater million rpm or whatever was said. the reason it does spin to quater million rpm with the engine running under full load is the heat energy attempting to escape from the cylinders. "its not just air blowing over the turbine" its gas and heat energy, expansion.
It's gas, and gas alone. Yes, the heat produced in the combustion chamber makes for gas expansion - but it's still that gas that drives the turbo, and nothing else.
quote from corky bell, author, maximum Boost: Designing, Testing & Installing Turbocharger Systems (Engineering and Performance)
The heat in the exhaust, expanding the gases hugely. So there is "more gas" to drive the turbine. The heat doesn't drive the turbine at all, it simply provides a greater volume of gas.
ok? so we are now agreed the gas wouldnt be expanding hugely, and wouldnt be multiplying the force round the turbine independant to the source gas pressure, if the gas were cold or warm and pumping through at a linear rate. heat, not just blow! exactly what i said in my first reply, back when it was layman enough terms to be relevant to this thread.
No need to let it warm up, just be gentle till its up to temp. A turbo car needs a little time to cool the turbo IF its had a ragging, and then only for 20-30 secs. Turbos do spin as a result of heat, although being pedantic TSC is right too as its the speed and flow of the hotter molecules that promote more flow. However, a turbo will spin faster and harder under load at any given revs, than it would with the car sat still at the same revs - this is becuase of the increase in heat when under load. I'll throw this one in too - dont depress the clutch when you start the car. your loading up the crank, without a nnice flow of oil over the bearings.
Itis just blow - but the heat creates more blow by expanding the gas to a greater volume. That's the simplest way I can put it, if you can't grasp that then I'm afraid there is no point continuing...
My car always has sh*t loads of mayo under the oil filler cap (MK4 2.0). 90% of the driving I do is at 60mph+ and hardly ever for short journeys. Is this just an effect of the design with the filler on MK4's, or is my car protesting about something. Sorry for interrupting the discussion about heat causing gas expansion. Although, I'd love to discuss the Carnot heat engine and other thermodynamic topics, please.
