Greg Locock wrote: "Very interested in how you got an estimate of 150Nm? Did you/do you know the weight of the rotating assembly and the exact compression of the engine?"
0.016 kg m^2 just a WAG
Greg Locock wrote:Compression is an interesting one. Does an engine with the ignition off slow down more quickly with an open throttle or a closed throttle? If you /know/ stay schtum for the time being, if you don't, explain the reasoning for your answer and give a rough guess as to the size of the effect. How much does CR affect it?
You know I had never thoughtabout it! It sounds like one of those riddles is a man going deeper into the forest once he passes half way or is he on his way out??
Never the less here are my drunken pre-international flight thoughts!
Ok firstly a closed throttle would on first blush simply due to the fact the engine would be operating at its lowest available RPM that being idle. If the throttle was open by any % above zero throttle input which is idle speed, then the RPM would be above the idle value and therefore the time to stop should be greater by some determinable amount of time however infinitesimally small. Compression would therefore not be the determining factor, RPM value would be as a result of the increase in the time taken to go from (idle+x)RPM where "x" is a result of a % of throttle input than the time taken to go simply from (idle)RPM where throttle input equals zero %.
However, i think what your trying to actually get at which is if you switched off an engine and then either:
A: left it with zero % throttle input; or
B: a throttle input above zero %
would the increase in throttle % in effect an increase or decrease the time taken for the engine to stop.
So let's set some specifics.
The engine is at the same RPM prior to the switch off;
There is as close to zero time differential between switch of and throttle opening as possible and as such this value is zero
It's an NA engine, no forced induction.
The throttle %'s are either 100% throttle open or zero % throttle input (so what the engine sees at idle) at the time of switch off. So.......
If closed it should be pulling a vacuum against the closed throttle butterfly(s) where valves are open on the intake stroke which would create resistance to motion. However on the compression stroke the cylinder would then be seeing less volume than where the throttle is 100% open as the intake stroke was performed under restricted flow which causes an apparent vacuum. This vacuum should be relative to what the engine pulls at idle so there should still be enough pressure from charge to cause some resistance due to compression.
If the throttle is 100% open then it is drawing down against a lesser restriction on the intake stroke so there is less resistance but not zero resistance to the intake stroke. However it should also have a drawn a larger intake charge volume which requires more work to compress and provides more resistance during compression. Any resistance seen should be in someway relative to the vaccum seen during steady state RPM operation.
There are a number of other variables such as inertia of rotating assemblies, number of cylinders etc, but I am struggling with 9 beers to form any opinion other than it would likely be close to a stalemate.
How much effect does CR have? It determines the amount of force that will be required to compress the intake charge but does not effect intake restriction as that comes from valve area and throttle. On first review, I would have to say CR makes a difference as shown anecdotally by the high oppression engines vs. the forced induction I run. The high CR NA engines takemore to turn over at start and always seem to stop dead on switch off where as the lower CR turbo (not crank driven supercharged) engines are easier on batteries and starter motors and lazier on switch off. If there was no compression then the engine would not be working against any opposing force excepting friction and intake restriction.
I need more time to think about it and a 12 hour flight should be perfect if I'm not sleeping, but if i had to make a (poor) guess and pick one where i couldn't simply convince myself it was a wash which i keep coming back too then throttle closed would be it. But I am in no way confident.
Greg Locock wrote:On a directly related note, if the engine is running at 19000 rpm in the garage roughly what MAP is it running?
As this was a number of years ago an allowable map when the max RPM value allowed 19,000RPM. Yes I am old-ish.