8 replies to this topic

[kos]

I'm no engine expert, so please enlighten me on the subject.

I guess that when idle, engine is only need to produce enough energy to compensate for internal loses, drive auxiliary systems and have enough kinetic energy stored in the rotating parts to complete next compression/combustion phases.

So my question is why idle RPMs of the racing engines are so high?

Thanks

[Wolf]

I'd say because they try to have as small flywheels as possible (hence the greater energy for compression &c), and secondly because every little bit of that engine is optimized for combustion and operation on high revs, which means that they need more revs to produce same amount of energy on lower revs as normal engine...

[H. Eckener]

Kos,

One of the things to keep in mind is the fuel injectors. Many of them operate at very high pressures typically upwards of 30 barg, the mass flow across the orfice on injector is related to this pressure drop. So in other words with a lot pressure you can move a lot gas. Combine this with the fact that these injectors actuate in a finite amount of time, meaning they do not open instantly nor close instantly.
The minimum pulse width, duration of injection pulse, is around 0.5 to 0.9 msec for the kind of low impedance peak and hold injectors used in auto racing. As a side note the injectors in your car will operate at minimum pulse width around 1.8 msec, they can operate a tad bit lower, but typically don't because of consumer tastes, like a smooth idle.
Even though both can operate at these low pulse widths, all injectors will suffer greater mass flow variation, due to various electrical and hydraulic interactions at these low operating speeds. Variations can be upwards of +/- 10% on the fuel mass flow. This contributes to eradicate engine idle.
But back to the high rpm idle, we have a very high fuel mass flow at idle, even though we have short pulsewidth, and all engines require a reasonable ratio between fuel and air to operate. To operate around this ratio, you have to bring the quantity of air up, an easy way to do this is just operate the engine at a higher rpm.
And as Wolf pointed out, the F1 engine has a very low amount of inertia and the F1 engine design is optimized high rev performance, the fuel injector placement, the bore to stroke ratio, the valve timing, etc all contribute to the "forced" operation at a relatively high idle rpm.

[jpf]

What would be a typical idle speed for a modern F1 engine?

[kos]

Thanks for explanation

2jpf: From what I've seen on BernieTV, it is around 6K RPM

[Frank R. Champs]

Hello,

Great post, H. Eckener, you really covered the issue of low pressure flow through a high pressure injector very well!

Let me add to it that airflow through the engine also has a lot to do with the high idles of an F1 engine. The valve timing profiles for an engine that revs in the range of 17,000 rpm must be such that there is a high valve lift, and a long duration of valve opening, simply to make sure all of the airflow through the engine's intake system can make it into the cylinder before the valves snap shut.
As it turns out, though this cam profile is optimized for high engine speed, at low engine speeds, the exact opposite is required for maximized cylinder filling. Because an F1 engine spends most of its time at high rpm, the cam profile is voiced for very high lift and long duration. This means, at low engine speeds, not a lot of air can fill the cylinder for combustion, and combustion becomes very difficult to sustain, if not impossible. That's part of the reason why an F1 engine's idle must be higher, to make sure enough air is filling the cylinder for combustion.

BTW, I think that 6,000 rpm sounds a little high for an F1 idle speed. I have heard speeds in the region of between 2,500 rpm and 3,000, at the most.

Thanks,
Frank

[H. Eckener]

Hello,

Yes thanks for discussing the valve timming issue in more detail, as you were alluding to, the timing is optimized for higher rpm. In one specific area this means intake valve closing timining. Typically in engines the intake valve closes a little bit after BDC, that means the piston is already coming up before the intake valve closes. This closing time can be very significant in high rpm high performance engines. The reason this is done is to improve volumetric efficency and cylinder filling at the high end. Intake air has a certain amount of momentumn in it when it is moving and even though the piston is rising for compression, some of this momentum or ram effect can be used to fill the cylinders a little bit better, just prior to closing.
At slower rpms and with fixed intake timings, the momentum of the incoming air is less, and the filling can be significantly reduced, and even a reverse flow of air out of the cylinder could occur and probably does around their idle rpm or just below it, that would also explain why the idle so high. Obviously a variable valve timing system could reduce the issues associated with this problem.

[jennerabc]

Hey all,

Just a quick note on the actual idle RPM. I have worked with an IndyCar team for a few races, and yes as the last guy said the idles are more around 1 500 RPMish. And to warm up in the morning or before each practice they would rv to about 4 000 RPMish.

Jenner

[jennerabc]

But F1 cars may be alittle different, would not be much though.

Jenner