Engine mixture question

[SlipStream77]

Perhaps someone can explain to me why it is that engines rev higher/produce more power when more air is added to the mixture?

Since the fuel provides the chemical energy why is it that when engines run lean the revs rise. I have experienced it in a small 2 stroke engine and I have heard that dragster engines should not be allowed to run out of fuel when running on nitro methanol for what I think were similar reasons.

Presumably nitrous is used primarily to increase the oxygen levels and attain a very lean mixture and thus high power output?

I know things get hot when this is done, but why don't most high performance engines run very lean mixtures (once warm) in order to get maximum power? I would have thought the technology is available today to deal with the resulting temps?

Thanks.

[Mr2Mike]

SlipStream77 said:

Perhaps someone can explain to me why it is that engines rev higher/produce more power when more air is added to the mixture?

Since the fuel provides the chemical energy why is it that when engines run lean the revs rise. I have experienced it in a small 2 stroke engine and I have heard that dragster engines should not be allowed to run out of fuel when running on nitro methanol for what I think were similar reasons.

Model 2 stroke glow plug engines by any chance?

Engines with high specific outputs often require an excess of fuel in the mixture in order to cool the engine and in the case of model two stroke engine, it also improves lubrication (as the engine lubricant is carried in the fuel). In a spark ignition engine, an overly rich mixture hurts power output because the excess fuel displaces air, and because it burns more slowly than a optimal mixture for power.

In a glow plug engine a rich mixture cools the glow plug which retards the ignition timing even further, so the mixture has an even greater effect on these engines than on spark ignition (where the ignition timing can be varied to optimise power). This explains why the performance of a model 'nitro' engine drops off significantly as the mixture is made richer, essentially you are controlling the ignition timing by varying the air/fuel mixture. Leaning out the mixture improves performance and allows the engine to rev much more freely, but higher RPM = more heat, leaner mixture = less lubrication and engine = seized very quickly.

The same applies to the dragster engine, and in fact any engine that requires a rich mixture to provide cooling such as many (probably most) turbo charged spark ignition engines.

[SlipStream77]

Mr2Mike said:

SlipStream77 said:

Perhaps someone can explain to me why it is that engines rev higher/produce more power when more air is added to the mixture?

Since the fuel provides the chemical energy why is it that when engines run lean the revs rise. I have experienced it in a small 2 stroke engine and I have heard that dragster engines should not be allowed to run out of fuel when running on nitro methanol for what I think were similar reasons.

Model 2 stroke glow plug engines by any chance?

Engines with high specific outputs often require an excess of fuel in the mixture in order to cool the engine and in the case of model two stroke engine, it also improves lubrication (as the engine lubricant is carried in the fuel). In a spark ignition engine, an overly rich mixture hurts power output because the excess fuel displaces air, and because it burns more slowly than a optimal mixture for power.

In a glow plug engine a rich mixture cools the glow plug which retards the ignition timing even further, so the mixture has an even greater effect on these engines than on spark ignition (where the ignition timing can be varied to optimise power). This explains why the performance of a model 'nitro' engine drops off significantly as the mixture is made richer, essentially you are controlling the ignition timing by varying the air/fuel mixture. Leaning out the mixture improves performance and allows the engine to rev much more freely, but higher RPM = more heat, leaner mixture = less lubrication and engine = seized very quickly.

The same applies to the dragster engine, and in fact any engine that requires a rich mixture to provide cooling such as many (probably most) turbo charged spark ignition engines.

That's interesting, thanks it answers my question. I didn't realise the part that the mixture played in terms of cooling.

Incidentally, it was a 2.5cc nitro RC car engine, revs to about 20,000rpm.

I wonder if anyone has tried running an engine using liquid oxygen injection. It would provide a great deal of cooling, and a lean mixture. I expect it would just be too dangerous, but I'm sure the output would be good, at least for a few seconds.

I'm a little surprised that cylinder head cooling technology hasn't really moved on considering it's involvement in attaining higher power levels. Water injection seems to have gone out of fashion now.

[rev-erend]

Hey - you could run the engine on just compressed air .. if you wanted.. or just liquid oxygen.. just don't smoke near the exhaust

[BB-Q]

Liquid oxygen has been tried in drag racing in the 50's or 60's. It didn't work too well. Nitrous Oxide tends to be used as the Nitrogen-whilst not combustible- cools the cylinder, allowing for more power. Lean mixtures are a big no-no in the search for spark ignition controlled engine power.

Dragster engines running on Nitromethane use an air/fuel ratio of around 1.8:1 compared to a petrol engine's 14.7:1. The Nitromethane engine is on the verge of hydraulic lock- but what would you expect when you need 10,000bhp from a 2v per cylinder pushrod actuated 8.3l engine based largely on a design dating back to the 1960's?

[SlipStream77]

BB-Q said:

Liquid oxygen has been tried in drag racing in the 50's or 60's. It didn't work too well. Nitrous Oxide tends to be used as the Nitrogen-whilst not combustible- cools the cylinder, allowing for more power. Lean mixtures are a big no-no in the search for spark ignition controlled engine power.

Dragster engines running on Nitromethane use an air/fuel ratio of around 1.8:1 compared to a petrol engine's 14.7:1. The Nitromethane engine is on the verge of hydraulic lock- but what would you expect when you need 10,000bhp from a 2v per cylinder pushrod actuated 8.3l engine based largely on a design dating back to the 1960's?

Thanks for that, that's interesting. I thought someone would have tried it!

Cool Volvo by the way, my dad used to have a 245DL, when the exhaust broke, it bypassed the rear silencer and on the overrun it sounded fantastic! Incredible amounts of popping and banging as unburnt fuel detonated. If I'd heard it and not seen it, I would have thought it was a Can Am car, all from a 2 litre 4 cylinder

[theshrew]

Slipstream. A Rc engine uses the fuel to burn cool and lubricate. The more you lean them off the quicker they will be but you can go to far with them and they sieze up.

Basic rule of thumb for any engine is the more air you can get in the engine to burn the more power the engine will produce. This also has a limit cant think what the fuel to air ratio is off the top of my head tho.

What are you actually trying to find out about RC engines ? I presume you are looking for more power ? There are a lot of things you can do to get the engines to run quicker. Polishing the ports different exhaust pipes etc etc but some things your not aloud to do if your racing so you would have to read up on the rules if you are.

[CNHSS1]

Glow motors also have a taper liner in which the piston runs to give a gas tight seal rather than a piston ring. the system is generally known as ABC standing for Alumininum piston, Brass liner which is Chrome plated. the theory is that the brass liner expands more than the piston so that cold the piston 'sticks' in the liner, but as soon as its running the expanding liner maintains a tight fit.
as mentioned the fuel contains the lube, so ultimately too lean could possibly seize the piston into the liner, but more likely it will just wear the hard chrome off the liner and result in negligible compression.
i used to work in the industry and did a lot of R&D for the manufacturers and fuel suppliers. changing the fuel makes a big performance difference, choosing race oils and greater nitromethane content will add big chunks of performance. a tuned pipe suitable for the motor and its usage i.e. off road or circuit racing etc also makes a huge difference.
the other point is that from 2.5cc, 20,000rpm isnt that hot these days, when i last worked with them, we were pushing 40k+rpm from 2.5cc and 43k+rpm/2.4hp from the larger 3.5cc engines on 25% to 40% nitro ;-)
CNH

[SlipStream77]

Thanks guys, I was using the RC engine as more of an example than anything else, Mr2Mike answered my question by explaining the cooling effect of the mixture.

With the drive to improve efficiency and economy these days, I expect cylinder head cooling to become more advanced in order to be able to run even slightly leaner mixtures.

My dad reminded me that Mercedes Benz developed and tested sodium cooled valves back in the 50's or 60's, anyone know if this or anything similar is still being used?

[annodomini2]

SlipStream77 said:

Thanks guys, I was using the RC engine as more of an example than anything else, Mr2Mike answered my question by explaining the cooling effect of the mixture.

With the drive to improve efficiency and economy these days, I expect cylinder head cooling to become more advanced in order to be able to run even slightly leaner mixtures.

My dad reminded me that Mercedes Benz developed and tested sodium cooled valves back in the 50's or 60's, anyone know if this or anything similar is still being used?

A lot of turbocharged cars run Sodium filled exhaust valves.

[CNHSS1]

Some (poss all) nissan turbo motors use sodium valves.

back to glow motors 60-70% of the engines cooling comes from the fuel rather than the cranckcase/cyl head finning

[SlipStream77]

Thanks guys, once again, PH provide all the answers!

[Huff]

IIRC the reason that engines don't run super-lean mixes is the NOx emissions.

In effect, the decision was taken to burn more fuel (based on maintaining lambda=1, ie a stoich mixture) and clean up the exhaust better, by using catalytic converters, than to allow the pursuit of super lean burn technologies for fuel economy alone. Certainly Honda amongst others demonstrated petrol engines capable running of AF of 22:1 at cruise...

[2woody]

don't forget that the above only applies to nitro and glow-plug engines.

a four-stroke running on gasoline requires a slightly-richer-than-stoichiometric mixture to produce the best power.