Knock Threshold Theory 1

[Turbinator]

I posted this in the combusion/detonation forum but the last post there is a week old:

For a turbocharged engine:
What is the relationship between the knock threshold and air pressure? My question is related to running the maximum amount of boost on pump gasoline. I know that as air temperature increases, so does the tendency for engine knock. Independent of temperature, does increased pressure lead to engine knock? For example, say you are running a pressure ratio of 3:1, but it is intercooled down to ambient temperature, would the limiting factors for knock (ignition advance,compression ratio,ect) be the same as for a normally-aspirated engine, all else being equal? Is there any data on this subject? Thank you.

 

[JRW261]

Humidity has effects on knock as well, just thought I would throw that in there

 

[Azmio]

Turbinator,

Knocking phenomena can be divided into different categories. However, all the knock categories are somewhat related to heat.

Now it may help a lot if we can supply lots of air to the engine without any temperature increase. However, if we look at the PV/T relation, it is hard to get rid of temperature increase once we raised the pressure or we decrease the volume. It is a necessary evil.

I also have not seen any intercooler that is capable of lowering the intake charge temperature down to room temperature. If you have such intercooler, I would like to know more about it.

As for the NA engine, the highest ve for the production car engines that I have worked with is about 107-108%. F1 and sport bikes engines get more. The production car engines intake temperature is not far from the ambient temperature, therefore the knock tendency is slightly less if compared to turbocharged. For F1 and sportbikes, the airbox is pressurized due to the ram air effect, still the temperature increase is not as high as the turbocharged one. Anyway, to answer your question, yes the ignition timing, compression ratio, etc are still the limiting factors even for NA engine.

 

[patprimmer]

Azimo

If the compressed air is cooled in an intercooler, then fuel is added (especially if it is alcohol) or water is added, or sprayed on the outside of the last stage of the intercooler, ambient or lower temperatures could be reached.

Turbinator

this site might show some original research that answers some of your questions.

http://naca.larc.nasa.gov/

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Azmio]

Pat,

Have you got the intake charge temperature before it gets into the cylinder, it will be interesting to see the reading. My guess is that it will be somewhat higher than the ambient temperature even if we managed to get the incoming air closer and closer to room temperature. The intake port is surrounded by water jacket and the coolant is about 90c during engine running. Heat transfer involves.

Anyway, same thing with the fuel or alcohol sprayed to the incoming air, the fuel itself experiences temperature increase as soon as it gets into the engine compartment from the fuel tank.

Overall, I am really interested to find out technologies or know how available that can reduce the compressed air close to ambient temperature. This will give new dimension to the road and race cars.

By the way, the ambient temperature mentioned here is the one sucked from the front of the car, therefore it has such a low temperature if compared to the one sucked inside the engine compartment.

 

[Turbinator]

Pat- Thanks for the site! Lots of good stuff there, I'll be combing through it.

You can cool air below ambient temperature using an air/water intercooler with ice, or dry-ice. I just used "ambient temperature" as an example, since I'm basically trying to find out if the only reason you have to run lower compression or retard the timing on a turbo/supercharged motor is because of the heated air.

 

[patprimmer]

Azmio

I think if the intercooler is very efficient, and then a liquid is mixed with the charge, or sprayed on the outlet end of the intercooler, the latent heat of vaporisation can reduce the charge to less than ambient

To help keep this temperature down as the charge re-enters the engine compartment, the air ducts and manifold should be made from insulating materials, and further insulated by thermal wrap or whatever.

This strategy works a lot better for methanol fuel, as it has a much higher latent heat of vaporisation and requires a greater mas of fuel.

For short duration power applications, the whole system can be cooled by use of ice or dry ice.

To go one step further the use of methanol as the only fuel, and nitrous oxide injection, can result in the charge being cooled to the extent that misfires develop.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[obanion]

Did anyone notice that the original question has not been answered? This became a intercooling discussion.

My understanding is knock comes directly from temperature. Now, other factors in turn effect temperature, but it's temperature the directly effects knock. Does the fuel:air mix at any point reach auto-ignition temp before you want it to?

Compression ratio, AFR, and intake temperature are the main influeneces, with some smaller ones (leftover exhaust gasses) also having a influence.

Intake temperature is important as it's the starting point. It only gets hotter from there. Lower starting point, lower temps at every stage of the process. So, keeping the intake air charge low aids knock prevention.

Higher compression ratios cause more compression, and therefore, more heat during the compression cycle. This again raises the air:fuel mix closer to the dreaded auto-ignition temperature.

Finally, the actual power output for a single combustion event (in terms of TQ not HP), has a effect. When the spark ignites, the flame front starts to burn through the total charge. As the center burns outward, the outside region of the air:fuel mix is compressed even further by the pressure generated by the onset of combustion in the center. I'll refer to this as "inter-combustion compression." This raises the temperature of the air:fuel mix on the outside of the cylinder. This is usually how knock starts, this region of air:fuel mix will ignite, and now you have 2 or more flame (and pressure) fronts colliding with each other. Once it begins, it tends to get worse, as the knocking causes lots of heat which sticks around for the next cycle, and on, and on.

So to answer the question, for two motors, even if you have the exact same compression ratio, and intake temp, and one running at 14.7psi (2BAR absolute) and one being NA, the turbo one should still require more octane, because of factor of cylinder pressure. More air:fuel in the cylinder, more power, more pressure, and therefore more "inter-combustion compression."

 

[patprimmer]

obanion

The Link to NACA gives enough info for the original poster to pretty well answer the question for himself.

Also the question supposes that a boosted engine can obtain the same charge temperature as an NA engine, so whether or not this is only hypothetical or is in fact real, has a bearing on future answers.

Also, the original poster gets involved in the intercooler discussion in a way that shows he thinks it's relevant to his original post, so why not let him decide what's relevant.

Also, you have asked some very theoretical questions with some fairly hypothetical conditions in some other threads. The NACA link might also be of interest to you. The NACA was the precursor to NASA, and is a part of the NASA site. It documents among other things, the research into aircraft piston engine development during WW11, which was a very active period of piston engine development.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Turbinator]

On the NACA site is a great paper:

http://naca.larc.nasa.gov/reports/1946/naca-tn-1131/naca-tn-1131.pdf

which charts the effect of Compression Ratio, Spark Advance, Inlet Air Temp, Coolant Temp, and Engine Speed against knock-limited IMEP (Proportional to torque) for 3 different fuels.

Does this mean they raised the inlet air pressure until it started to knock?