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Intakteair Temperatur and knock

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Hallo,

in the course we have learned that the ecu reduces fuel with rising intake air temperature since there is less oxygen in the same volume.

How much influence does a high intake temperature have on knock?

A lot -- a rise in combustion temperature will lower the knock threshold. Intake Air Temp is a big part of the combustion temperature. Increased Head Temp (coolant temp represents this) is the other big influence.

So on a knock limited engine would it be a strategy to use intake air correction to let the engine run richer on high intake temp?

So on a knock limited engine would it be a strategy to use intake air correction to let the engine run richer on high intake temp?

One could, but it would be well down on my choice of 'remedies' as it introduces it's own problems - primarily bore wash which will increase ring, bore and piston wear while also diluting the engine oil and causing other issues from the compromised lubrication.

In no particular order, and I expect I've missed some, the options would be...

Live with it.

Use a better fuel and/or octane boosting additive and/or water/methanol injection/spray. The last has the advantages of both lowering the air temperature and increasing the knock threshhold - just DO NOT allow it to run out unless you have some other protective measures in place to back it up.

Improve the charge cooler and/or engine cooling to reduce charge and/or engine temperatures as they can have a significant affect.

Rebuild the engine with lower compression.

Or a mixture of the above.

Thank you.

It's about stock class racing so changing the cooling or compression ratio is not an option. It's about a leaner start for best power and richer on hot engine for safety.

Thank you.

It's about stock class racing so changing the cooling or compression ratio is not an option. It's about a leaner start for best power and richer on hot engine for safety.

There's stock, and there's "stock"... Have a careful read of the rule book as sometimes there's provision for replacement parts, sometimes areas that aren't regulated*, if you think you've found an advantage have a work to the scrutineer to see if it's allowed as he/she will have the final call. NOTE, different scrut's may have different interpretations, so it you're competing in different places, be prepared to change it back.

If it is stock, though, I'm a little puzzled why it's knock limited in stock form? I don't know how well regulated fuels are over there, but I would expect some retailers to be a bit better than others - some modern fuels don't store well, so perhaps if you get your fuel from a busy service station where the fuel is restocked frequently, rather than a station where the fuel may sit in the tank for a while? You may have the option of buying fuel in drums as some race series allow that, use a control fuel or even, if you're lucky, an ethanol containing fuel such as E85.

If it's for the jetski, and it uses a thermostatically controlled heat exchanger, you may be able to use a lower temp' thermostat, heat shielding, improved airflow through the engine compartment, etc. It may not only allow more timing, but a denser charge :-) As part of your maintenance and pre-race checking, don't forget to make sure ALL the water channels are clean and free of crap or corrosion - if you're operating in salt water, this can be critical - make sure your anodes, etc, are in place and have material left.

*For example, it's common to paint engines for corrosion protection, or just looks - strategic use of heat barrier paint(s) may be perfectly legal and make a small difference, and it's a series of small differences that add up to the winning advantage.

Forgot, I'm assuming you've increased boost - within the rules, but sometimes this is counter-productive and less boost will lower charge temeratures and allow more optimum timing - and this can actually mean more power...

Hi Gord,

Thank you for the recommendations. I'm running race fuel on that unit. However, racing is more than machine. I just totally messed up the world cup in all classes.

One consideration regarding intake air temperature if the air gets hotter Ecu injects less fuel, engine produces less power and less heat.

So shouldn't this compensate the total heat amount and not really have influence to knock?

Hi Gord,

Thank you for the recommendations. I'm running race fuel on that unit. However, racing is more than machine. I just totally messed up the world cup in all classes.

One consideration regarding intake air temperature if the air gets hotter Ecu injects less fuel, engine produces less power and less heat.

So shouldn't this compensate the total heat amount and not really have influence to knock?

Hi Gord,

Thank you for the recommendations. I'm running race fuel on that unit. However, racing is more than machine. I just totally messed up the world cup in all classes.

One consideration regarding intake air temperature if the air gets hotter Ecu injects less fuel, engine produces less power and less heat.

So shouldn't this compensate the total heat amount and not really have influence to knock?

Sort of, the air is hotter, therefor less dense at the same pressure and so it will indeed normally require less fuel - this is partly why modern vehicles run at higher temperatures. However, the hotter air will be more easily ignited and not only burn faster but may be more susceptible to ignition in areas where it is being compressed, often by pressure waves in the chamber causing compression ignition (like a diesel engine uses). Rather than decrease the fueling, sometimes additional fuelling is provided to try and reduce the charge temperature by evaporative cooling.

As you said, while there are other considerations, power is approximately proportional to the amount of fuel that is burned in the engine - not the amount supplied, the amount used. This is why it is possible to make more power by reducing boost and the resulting air temperature gain. There are many instances where engines have lost power when 'boost' has been increased, because it was temperature and not density that was increased by the higher boost.

If you've looked at the other options, then I would certainly suggest dropping the 'boost' and correcting the fuelling as required (the engine may respond better to a little more, or less, than you're using), and ignition timing so you are just short of the point where you are 'knock limited'. Then you can try increasing it slightly to see where the 'sweet spot' lies.

If you don't have one, I'd suggest picking up a GPS based speed and acc'n meter that is suitable for use on a boat, so you have an objecte rather than subjective feedback on changes.

Thank you. I got a proper GPS but can't really use acceleration measurement to compare settings. The differences are to small and within the tolerances of different runs