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Old 24th December 2018, 15:05   #406
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Default Re: Why you must practice the "Idling Rule" with Turbo-Charged Cars

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Originally Posted by vharihar View Post
I am no auto/mech engr, but my gut feel told me all these days that the engine rpm is a *proxy* for the "loading", by means of the transmission. In other words, whether you end up revving the engine at 2500 rpm by driving in the 5th gear at 90 kmph, or at the same 2500 rpm by driving in the 3rd gear at 40 kmph, I thought that the engine loading was the same in both cases (since the rpm is the same).
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On cars the engine RPM do not say much about how much power the engine is generating perse. An extreme case, think about diesel generators. They run at constant RPM and generate from idle load to max load all at the same RPM.

The actual loading of an engine is a result of the engine having to overcome resistance. During acceleration it is more about having to accelerate a force obviously. But at a steady pace driving it is about overcoming resistance. By far the greatest one is air resistance. If you drive at 2500 RPM in first gear your car might be doing 15-20 km/h if that. At those speeds air resistance is very very small. But you do 2500 RPMin fifth gear you are likely to be doing 90-110 km/h. Air resistance is exponentially higher. What it means at the very same RPM of the engine is loaded up considerably more. (And thus much higher exhaust gas temperature

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Originally Posted by Rahul Rao View Post
@vharihar.
Actually with low load engine is running at lower compression l
Compression or rather compression ratio is fixed on an engine (well almost any engine). The compression ratio is determined by the cilinder diameter and the stroke of the piston. They are, obviously, fixed.

The actual compression can be to some degree influenced by valve timing. The actual compression only starts when both valves are closed.

Especially on turbo engine the intake air is somewhat compressed (by the turbo) that means it is also warmer as it enters the engine and it has a higher density. When you compress warmer air, the final compression pressure and temperatures will go up as well, compared to colder air. The higher the load the higher the inlet air temperature and air density.

(It is one of the reasons most turbo engines will have somewhat lower compression ratio’s compared to none turbo engines. or even intercoolers)

Jeroen

Last edited by Jeroen : 24th December 2018 at 15:07.
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Old 24th December 2018, 15:19   #407
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Default Re: Why you must practice the "Idling Rule" with Turbo-Charged Cars

@Jeroen
The physical compression ratio is fixed. But the turbo boost varies depending on many factors. If you are going down hill in gear without touching the throttle, the air entering engine is reduced, as (in fixed geometry turbo waste gate is bypassing the turbo) and in VGT the vanes are in to push in far less air.

Along with this no or negligible amount of diesel is being injected.


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Old 24th December 2018, 19:06   #408
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Default Re: Why you must practice the "Idling Rule" with Turbo-Charged Cars

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Originally Posted by Rahul Rao View Post
@Jeroen
The physical compression ratio is fixed. But the turbo boost varies depending on many factors. If you are going down hill in gear without touching the throttle, the air entering engine is reduced, as (in fixed geometry turbo waste gate is bypassing the turbo) and in VGT the vanes are in to push in far less air.
We are discussing ways to cool down the turbo. Boost pressure is not a good measure for what is happening on the turbine side of the turbo. That is where the hot exhaust gasses are. Engine load has a direct correlation on the exhaust temperature.

There is of course some correlation between boost and exhaust temperature and volume of exhaust air. But with waste gates it is way more complicated. Heck, even without a waste gate it is more complex.

If you want to know how long you need to cool down a turbo, you need to know how hot the turbine is. Which is directly related to exhaust gas temperature and how long that has been going down that turbine. Cooling down, same thing, how much air at how much temperature determines how much and how fast you cool it. All down to engine loading, nothing else really.

But to put your comments to practical use:

If you own a turbo engined car, make sure your house is at the bottom of a hill, so you can essentially coast home the last few kilometers. You will never ever have turbo problems!

Jeroen
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Old 25th December 2018, 09:42   #409
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Default Re: Why you must practice the "Idling Rule" with Turbo-Charged Cars

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Originally Posted by Jeroen View Post
But to put your comments to practical use:

If you own a turbo engined car, make sure your house is at the bottom of a hill, so you can essentially coast home the last few kilometers. You will never ever have turbo problems!

This is something I always do.

I'm half way up a gradient, so I always return home from the the higher side with engine idling in a mid gear. In fact I don't have to touch the throttle for over a minute when I approach or depart from my home.
I have never faced turbo issue even in cars as basic as 2005 Indica turbo whose engine lasted till car was sold after use of 3.24 Lakh km.
Even the retrofit turbo in my Dad's 1989 Contessa lasted from 1993 to 2004.


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Old 2nd January 2019, 07:07   #410
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Default Re: Why you must practice the "Idling Rule" with Turbo-Charged Cars

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Originally Posted by Jeroen View Post
......Initially there were some concerns whether these self contained system would be able to ensure sufficient lubrication when the engine was switched off. Due to over pressure in engine rooms, the turbo’s tended to spin, slowly but surely, on differential pressure between the engine room and the outside.....Jeroen
Perhaps I am ill-informed - and it would be interesting to know the source of these "initial concerns", since I have yet to come across / read about an example (post late-60's, since I stepped on board my first vessel in 1973) incorporating both the gravity feed system & also the self-contained lub. system for T/C emergency lubrication. It would almost appear to be a case of over-kill!
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