Compression:Pressure rise:temperature

[errolc]

Hi Guys, I'm new to this forum & discovered your site searching information on ceramic coating (piston,head & ex port) for a project building a hotsaw (chainsaw)..I'm a bit of a petrol head in that Its been my hobby for 30 odd years now to performance tune most things I get my hands on and I've become pretty good at it (karts, bikes, cars & now chainsaws) for myself and quite a few others over the years..just read Erics post on nikasil & plating iron sleeves..can relate the the lack of logic and knowledge out there..have been trying to source here in NZ where I can get carbide bits sharpened through electrolitic erosion (electroplaters shouldn't have any problems doing it) but do I get some blank looks when I ask about it..

Anyway back to my topic. I'm searcing for information (prompted by a question put to me) whats the minmum temp you can run a 2 stroke at and attain max output..and you know I hadn't really experimented with it..just pretty much done the usual thing and accepted what the rest of the fraternaty have done (without question really) and have usually got better results through careful attention to detail.

I have test information that meassured performance gains down to 60C..this was external readings..and it has pricked my curiosity as to what the min operating ingnition temperature would need to be..based on the flash point of fuel and the compression pressure rate rise in temperature, and considering the temperature requirements of libricants and the advantages of cool intake charge etc...I know there is a formula/table that predicts rate rise..& I know at 24:1 compression ratio at 2500rpm charge ir temp is 800C from the Bosche diesel fuel injection tech manual..the argument has been pusshed to me "you don't want the engine too cool"..my argument (for air cooled engines) is water cooled engines run far cooler surface temperatures internally..but how cool I can't say..any ideas or help on the topic..

I run a little test a few years ago on a 50cc blower, coated the piston,head & ex port with VHT paint (properly prepd) and saw a 40-50C drop in head temp and about 500rpm increase which to me represented a power increase given all other things were equal and it certainly sounded a lot crisper at the end of test runs, enough to convince myself and Ive just used it as a matter of course ever since, but now I'de like to do some calcs 1st, and experimenting just what the gains are

 

[captblue1]

exactly me and my girlfriend were just talking about that the other night. huh wierd.

 

[usgpru27]

On our Grand Prix Bikes we have found that differant manufactures have differant sweet spots from a temp stand point. A Honda RS125 makes max. HP at 50c but will slightly fall off at 55c but carborate much better. Our Yamaha TZ125 makes max at 65c. I would think that your paint experiment didnt really make it run any cooler, and is a heat transfer barrier not allowing as much heat to escape via the surface area. This in turn is rising combustion temp and may not be a bad thing. I would assume that a blower is set from the factory to be rich and have more than sufficiant cooling so that they dont blow up. Possibly you test mearly incresed the engines efficiancy. I would guess extended usage would shorten the life of this engine.

On another note. I took the head off my CR125 last week and could not believe the small size of the bleed holes for coolant in the head gasket. A thermostat of sorts. Has anyone increased these hole sizes for the hot summer to increase cooling? Certainly slower warm up but no big deal. I was thinking of this after going up the Turtle trail at Paragon 2 weeks ago when it was like 95 out. That engine must have been roasting at the top. I am afraid to put a temp gage on it. Any thoughts?

 

[errolc]

Thanks for your thoughts...the blower I used, I made principly for cleaning up around the house, the off the shelf blowers are a bit pathetic so I used a Johnsered 51 and tuned the fan capacity down to a bit below the engines peak revs..and thought at the time of the test the blower would work much the same as a dyno in absorbing the output in a consistant manner, through the displacement of the weight of air through it..I have been reading a physics book..Thermodynamic Analysis of combustion engines by Ashley S Campbell (today), and it provides theories supported by trials which seem to prove the cooler an engine can run, the better the performance output in HP terms, however emission go up..hence the situation we have in thinking of engines needing to be at operating temperature..Quote pg87:"chemically correct mixtures...cylinder temperature at the start of compression, from 328 to 305K increases power output from 42.6 to 46.0hp." 274deg K = 0 deg C so 328 = 53C then goes on to explain f effects of fuel evaporation dropping charge temp to feezing with ice problems.

another book "Two-Stroke performance tuning" provides similar evidence and conclusions..searching the web I found further material to support the same conclusion..that is..petrol/gasoline has a flash point about -45degC (the point at which it fumes/gasifies) and an auto ignition point about 260C (detonation) remember these are charge temperatures..my conclusion is that effective (not confused with efficient) ignition occures between these to limits, every thing I've read talks about efficiency in terms of complete ignition/emissions (top fuelers aren't efficient, but man can they make HP)..further reading suggests the lower limit is determined (for carbureted installation) by icing factors..in 2strokes its not a worry because there is no port of any length for this to become a factor..sooo my thinking is, the lower the operating temp, the cooler the fuel/air charge can be kept (there is a limit as to how cool an engine can be insulated from the heat) the higher the compression for a give octane fuel..higher compression translates into faster flame propagation (less advance) AND higher heat of ignition for greater BMEP numbers.

I've been trying to find information on the surface temperatures incurred through the ignition cycle but haven't been able to find anything specifically other than the heat of ignition peaks at about 4500f for less than a milli sec, combustion is over with 4 milli sec and this period is to short to transfer much of the temperature to the surface of the engines internals due in part to a shielding boundry layer..much of the heat is transfered during the after burn (the period after the initial flash where the real pressure buildup takes place) so the engines internal temps follow a more even wave temp path than the significant peaks and trouphs of the intake/exh gases..this was able to explain to me why the VHT paint (1500f) didn't burn away (other than a surface film of chalky material (seems to stand up very well considering)..think about it, the oil (which carburises about 400C) on the cylinder walls doesn't burn off..the idea with the ceramics is to prevent transfer of heat expanding energy from the combustion gases to the container (engine) but to keep it contained within the gases to maintain pressure..take a corked bottle and heat till it pops, chill it before it pops and you lose the pressure :-).

Further to this there is a balance achieved in the amount of swirl a charge can carry and be effective/efficient..the greater the swirl, the more effective (promotes faster ignition travel)..however to much and there is a lot of heat lost to the cylinder/piston & head from a significant scrubbing effect it has on that boundry layer (same as wind chill factor, in reverse - a blow torch effect)..logic tells me that if you can insulate the engines workings (at least the majority anyway) from the heat..this factor can be further exploited....

This goes further, in that the better the heat (& expansion issues) are managed, the tighter tollerances can be controlled with less ring losses...your thoughts

What I'm trying to do is rationalise and quantify my thinking in a logical direction, based on my own experiences...because I don't have access to dyno equipment to test my ideas, and its really starting to bug me to work it out...

 

[errolc]

oh..your bleed holes..be very carefull about opening them up..they are ther to control the flow around the head and cylinder to keep things even and avoid hot/cold spots...if you alter them, you WILL need to increase your pumps capacity to compensate for the changes in the direction and channelling of flow.

 

[usgpru27]

On our Grand Prix engines these passages are wide open. Actually we use O-rings so there is no restriction. We control heat with tape on the radiator. These holes work as you note as a thermostat. I am going to put a temp gage on it to see what it is running.

 

[errolc]

its interesting to note the variation between the Honda@50C and the [email protected]h one makes the more power/torque/rpm..are there BMEP figs available..I wonder how much swirl patterns have to play and what the differences are in their case temperatures/designs which would affect the charge temp and evaporation/charge homoginization..you say the HP drops off about 55 for the Honda..what about the other way..cooler running/weather?

 

[usgpru27]

You cannot run the bike much lower than 50c when tuning it very aggressivly. The piston expansion starts to be to great and with a cool cylinder you will have issues. Weather plays no part. You vary the amount of tape on the radiator to compensate for the weather. Target is 55c on the Honda. Max power is at 50c however as I noted it does not carborate as smoothly and ultimatly it is not as fast since you cannot get the power down when you want to at apex.

 

[errolc]

Interesting...seems somewhere between 50 & 55 is the threshhold for this engine..I've been reading about fuel volatility, that is the measure of a fuels properties to completely vapourise to create a homogenous mix with the air..there are three states to any fuel...flash point (when the fuel begins to evaporate off..self ignition (point of spontatious self detonation)..and the bit in the middle where the fuel turns from a vapour (as in steam droplets) to a gas...sooo you could run 2 fuels of the same octane with different levels of volatility and push that figure lower.

My thinking is this..by insulating (with ceramic, or in my case for lower performance 2 stroke engines) VHT paint...don't laugh..it lasts well enough between strip downs..by insulating the base material, cylinder head, piston top & under side, and exh port from the raw heat..the difference between ambient air temp and the engines working temp is significantly less and therefore more stable (less variance between max & min working temps, AND alowwing the engines mass itsself to be more heat stable. BUT..and this where a basic level of logic needs to be considered..the surface temp of the combustion internals should remain about optimum, without transferring as much heat to the crankcase (read ..air/fuel charge)..a cooler case means cooler/denser charge (above the point of condensing)..more power.

the way I see it..ceramic coatings (of what ever method) compared to metal surfaces, is more resitant to being heated. It will however heat up to comparable temps to a bare metal surface (takes care of the condensing issue) but it won't want to dissapate heat back into the next incoming charge (readily), because there is not much HEAT, mainly brief high surface temperature...remember the difference between heat, and temperature..you can have high heat, low temperature...depends on the mass holding/releasing the energy and the rate of release.

Because the ceramic material resists aquiring energy, as much as it resists releasing it, it would be logical to see that really only the boundry layer of combustion heat/temp is going to be affected by any heat transfer to the surface of the coating. Once the very thin surface layer of the coating approaches combustion temp, signifacantly less energy is transferred..so it stays where it belongs, with the combustion gases..same applies with the incoming charge..less energy transferred to the intake charge...now ..have you ever heard of a cold engine detonating...this is where my research is going...what is the compression ratio that provides the temperature rate rise...to allow fuel/air charge to gasify and stay short of self ignition...I know 24:1 @ 2500 rpm @ 20C ambient temp gives temperature rise of 800C...But I need to find the formula that I can create a table to work from as a starting point...for our 2 stroke engines at their rpm...because the quicker you raise the pressure, the more energy is being imparted on the gases in a smaller space of time, and the higher the temps for the same compression ratio.

Having said all that..the dynamics of the engine will change as well..that is the engines clearances and tollerances. That is to say the piston and rings will remain tighter, better seal, crisper power...thats my logic anyway.

 

[2strok4fun]

I think what you are looking for is reffered to as Boyls law. Calculates adabatic temp rise. I think I have that right, could be wrong tho.

I have read some SAE papers on low heat rejection diesles, they were taking a different tack and doing little cooling but were employing thermal barrier coatings.

I am running TBC pistons in my watercraft Rotax motor, I havent had the opportunity to fully test or side by side test with a similarly modified ski. Our skiis have run pretty much dead even, I want to see if I can pull him after a long run where the heat soak starts to sap some power.

 

[2strok4fun]

Nope, I was all wet. Boyles law assumes equal temperature. So much for memory. :coocoo:

 

[errolc]

What I'm looking for answers to is..same tack..other direction..they're looking for economy/efficiency through retaining heat to assit combustion and achieving higher temperatures which more totally consumes the fuel (diesel is not very volitile) within combustion...I'm looking for power (other end of the same ruler)...in minimising heat loss/transfer to help create a more stable platform (engine) and assist in burning as MUCH fuel as possible, through a denser charge, effectively and shielding the engine from as much combustion heat as practicable..because of the tortuous path the intake charge takes through a 2 stroke...there is plenty of opportunity for the charge/fuel to thoroughly mix and gasify both with turbulence and heat transfer from the case..the site below is touching on the same concept

http://www.aquamist.co.uk/rescr/faq/airmass/airmass.html

in doing this I'm searchinig for the parameters required for the objective...lowering the engines operating temp, allowing raising the compression ratio beyond accepted norms, or increasing combustion assiting swirl, or a combination of both (which ever is the most desirable)..through better managing a cooler & denser intake charge.