Discussion
So I was wondering, why do most cars idle at 900-1000 rpm? My car states in the handbook that it should idle at 950 +/- 50 but I can take the idle as low as 500 or 600 rpm by restricting airflow using the idle speed setting screw before the engine starts to "stumble". Surely it would be better to idle at the lowest speed possible as that would be the most fuel efficient.
Possible ideas I came up with:
500 rpm is not enough to drive the alternator.
500 rpm is not enough to have the oil pump maintain pressure.
500 rpm is not suitable for the ignition timing of the engine.
Possible ideas I came up with:
500 rpm is not enough to drive the alternator.
500 rpm is not enough to have the oil pump maintain pressure.
500 rpm is not suitable for the ignition timing of the engine.
Raize said:
500 rpm is not enough to have the oil pump maintain pressure.
This, and on cold days or with an old engine, crap fuel, blocked air filter etc it might cut out causing "reliability reputation issues"Also less likely to stall as you set off and more responsive on the throttle from 1000rpm
Petrolhead_Rich said:
Raize said:
500 rpm is not enough to have the oil pump maintain pressure.
This...I'm no engine designer, but I'd guess that idle speed is more a compromise between smoothness, flywheel weight and responsiveness (large flywheels allowing lower idle speed, but more sluggish response).
TheLastPost said:
Oil pumps are driven by gears, so surely you can gear them up to run at whatever speed you like (within reason) relative to crankshaft rpm?
I'm no engine designer, but I'd guess that idle speed is more a compromise between smoothness, flywheel weight and responsiveness (large flywheels allowing lower idle speed, but more sluggish response).
Yea, but there's a range, so if it was geared up to pump a lot at idle, it would be doing huge amounts at higher RPMs.I'm no engine designer, but I'd guess that idle speed is more a compromise between smoothness, flywheel weight and responsiveness (large flywheels allowing lower idle speed, but more sluggish response).
The alternator and aircon are probably the biggest factors, most car will automatically bump the idle speed up when the aircon is switched on.
I know with BMWs, they have about 4 different idle speeds, 1200 for very cold weather, 1000 on normal starts, 850-900 at middling temps and 600-700 when fully up to temp.
Automatics often change have different idles too.
A major part of it is the torque produced at idle, a small engine might not have a lot and would be easy to stall, bigger V8s can go lower.
I'm not sure what it would be like on a motorbike, but usually you should be able to gently let the clutch out on anything at idle and it'll start to move off.
TheEnd said:
Yea, but there's a range, so if it was geared up to pump a lot at idle, it would be doing huge amounts at higher RPMs.
The alternator and aircon are probably the biggest factors...
Oil pumps have pressure relief valves, too - they're usually producing maximum pressure by the time they're doing 2000rpm; anything above that and the relief valve opens.The alternator and aircon are probably the biggest factors...
Alternators and air con are driven off pulleys, so again can be stepped up to give whatever revs you like relative to crankshaft rpm.
The idea of over-revving ancillaries being a limiting factor appears to be blown out of the water by the fact that a Honda V-TEC engine that revs to 9K rpm idles at broadly the same speed as a old push-rod 4 cylinder engine that blows itself to bits at 5,500rpm, whereas by your argument the latter could run higher geared ancillaries and a much lower idle speed.
Think it will be down to harmonics to a point as well, a lot of engines as they turn lower may set up an annoying buzz, also the car has to idle the same at the top of a mountain and below sea level so there may be an element of allowing for that, possibly to avoid stalling for the average driver.
TheLastPost said:
Oil pumps have pressure relief valves, too - they're usually producing maximum pressure by the time they're doing 2000rpm; anything above that and the relief valve opens.
Alternators and air con are driven off pulleys, so again can be stepped up to give whatever revs you like relative to crankshaft rpm.
The idea of over-revving ancillaries being a limiting factor appears to be blown out of the water by the fact that a Honda V-TEC engine that revs to 9K rpm idles at broadly the same speed as a old push-rod 4 cylinder engine that blows itself to bits at 5,500rpm, whereas by your argument the latter could run higher geared ancillaries and a much lower idle speed.
It's not blown out of the water at all, having ancillaries rev higher than they need to wastes power and therefore fuel. On a Type R this is barely of consequence, but on the average modern "eco" car it could make a significant difference. 500 RPM on most high compression 4 cylinder engines will be sounding pretty rough and it will be extremely easy to stall. A 3 or 2 cylinder engine wouldn't idle reliably at that speed without an enormously heavy flywheel. Still, the average PH poster clearly knows better than manufacturers that have been developing engines for more than 100 years.Alternators and air con are driven off pulleys, so again can be stepped up to give whatever revs you like relative to crankshaft rpm.
The idea of over-revving ancillaries being a limiting factor appears to be blown out of the water by the fact that a Honda V-TEC engine that revs to 9K rpm idles at broadly the same speed as a old push-rod 4 cylinder engine that blows itself to bits at 5,500rpm, whereas by your argument the latter could run higher geared ancillaries and a much lower idle speed.
On the one hand you appear to be saying idle speed is dictated by ancillaries:
Mr2Mike said:
It's not blown out of the water at all, having ancillaries rev higher than they need to wastes power and therefore fuel...
But on the other hand you appear to be saying that it's down to engine balance and refinement/response vs. flywheel size?Mr2Mike said:
...500 RPM on most high compression 4 cylinder engines will be sounding pretty rough and it will be extremely easy to stall. A 3 or 2 cylinder engine wouldn't idle reliably at that speed without an enormously heavy flywheel.
So what are you saying, exactly?Mr2Mike said:
.. A 3 or 2 cylinder engine wouldn't idle reliably at that speed without an enormously heavy flywheel.
Oh, and as an aside, the 1.0 triple in my Toyota Aygo doesn't idle significantly faster (maybe a couple of hundred rpm) than the 4.6 litre V8 in my range Rover.Go figure...
The VTEC-E engine in my '95 Civic was specially intended to idle at 600rpm +/-50rpm. This is 100rpm lower than the fairly similar other Civic engines of it's day. This was done purely as a further fuel saving measurement as it was the special highly economical model. Allegedly it saves 3% fuel. So manufacturers DO know this...
AntiguaBill said:
This was done purely as a further fuel saving measurement as it was the special highly economical model. Allegedly it saves 3% fuel. So manufacturers DO know this...
Yes, of course... but reducing idle speed to reduce fuel consumption is different - in fact the complete opposite - to increasing idle speed to allow lower geared ancillaries (thereby reducing ancillary power losses at higher revs) which is what some people are suggesting is the limiting factor on idle speed.TheLastPost said:
On the one hand you appear to be saying idle speed is dictated by ancillaries:
You are clearly easily confused, but I will try to assist with your confusion: Mr2Mike said:
It's not blown out of the water at all, having ancillaries rev higher than they need to wastes power and therefore fuel...
But on the other hand you appear to be saying that it's down to engine balance and refinement/response vs. flywheel size?Mr2Mike said:
...500 RPM on most high compression 4 cylinder engines will be sounding pretty rough and it will be extremely easy to stall. A 3 or 2 cylinder engine wouldn't idle reliably at that speed without an enormously heavy flywheel.
So what are you saying, exactly?1)Running ancillaries at a higher speed than they need to be run wastes power.
2)Engines with fewer cylinders won't idle well at very low RPM
Note that there is nothing contradictory in these two statements.
TheLastPost said:
Oh, and as an aside, the 1.0 triple in my Toyota Aygo doesn't idle significantly faster (maybe a couple of hundred rpm) than the 4.6 litre V8 in my range Rover.
Go figure...
There is no "Go figuring" to be done. Your V8 would probably idle fairly happily at 500 RPM, your Aygo almost certainly wouldn't. The fact that the standard idle speeds are similar is irrelevant to what I wrote.Go figure...
Mr2Mike said:
You are clearly easily confused, but I will try to assist with your confusion:
1)Running ancillaries at a higher speed than they need to be run wastes power.
2)Engines with fewer cylinders won't idle well at very low RPM
Note that there is nothing contradictory in these two statements.
But which are you suggesting is the usual limiting factor on idle speed?1)Running ancillaries at a higher speed than they need to be run wastes power.
2)Engines with fewer cylinders won't idle well at very low RPM
Note that there is nothing contradictory in these two statements.
My take on it is that you can choose the speed that you run your ancillaries, relative to crankshaft speed, by selecting appropriate pulleys/gearing, but in any case ancillaries should be designed to work with the engine, not the other way around.
I'd be interested in your views, though.
Mr2Mike said:
Your V8 would probably idle fairly happily at 500 RPM
Then why does it idle higher? A deliberate attempt to be profligate with fuel?Serious question... I can't see a reason for setting an idle speed higher than is necessary to ensure smooth running of the engine.
You're certainly right about the Aygo not being happier idling any slower, though - even the extra load from switching on the air con makes it feel a bit lumpy.
I would guess its a balance between having to put big lumps of counterweight on any reciprocating/rotating parts to make them work smoothly at lower speeds which is counter productive in many ways.
Against making them lighter but running at a slightly higher,just as smooth but not unusable speed.
A lot of mechanisms,especially those with gears or cogs `chatter`at low rpm, if they don`t have the weight or momentum to throw them selves over efficiently,a direct path to wear or failure.
If you increase the weight of any moving part with a counterweight to assist it then everything around it needs beefing to cope,and so on...
Looking at how a belt or chain reacts to a lower than normal idle speed,flapping and jumping all over the place should be enough to tell you that it`s not right.
Against making them lighter but running at a slightly higher,just as smooth but not unusable speed.
A lot of mechanisms,especially those with gears or cogs `chatter`at low rpm, if they don`t have the weight or momentum to throw them selves over efficiently,a direct path to wear or failure.
If you increase the weight of any moving part with a counterweight to assist it then everything around it needs beefing to cope,and so on...
Looking at how a belt or chain reacts to a lower than normal idle speed,flapping and jumping all over the place should be enough to tell you that it`s not right.
Gassing Station | General Gassing | Top of Page | What's New | My Stuff