How to run an engine without spark?

[custom442]

Its possible to run an engine on pre-ignition, from reading a lot of smokey yunick's old stuff. His fiero is still up and running today. The concept is to start the engine normally, preheat the fuel to x degrees and keep heating and pressurizing the cylinders to the point of preignition, ridiculously lean ratios that should break a normal engine, and at some point cut ignition and the thing runs by itself off of extremely high combustion temps. Some things he used - 4 cylinders for balance, all iron parts for extreme heat/pressure, turbo for high AFR/combustion pressures

So my question is what kind of cam grind would allow an engine to simultaneously take the effects of preignition until combustion temps of the engine are so great it can roll out on its own? Controlling the AFR would allow the engine to keep running after ignition is cut, but the getting there relies on the cam, any thoughts?

 

[rustyroger]

Pre ignition was a problem for old engines that had built up carbon deposits in the combustion chamber, the carbon build up had two effects;
the compression ratio was effectivly raised simply because the combustion chamber became smaller, in reality this din't amount to much,
carbon glowing red hot ignited the fuel before the spark occurred - hence the term pre ignition, this would also cause the engine to continue firing without a spark when the ignition was switched off if the carburettor allowed fuel to continue to be drawn. Idle shut-off solenoid equipped and fuel injected engines didn't have this problem.
Diesels are compression ignition engines, that is as the air is compressed on the compression stroke it heats up sufficiently to ignite the fuel when it is injected. Typically the cr for a direct diesel is 15:1 (most big truck and industrial engines), 22:1 for indirect injection (as used on small engines for cars). That is why diesel engines are much more robustly constructed then gasoline engines.
It seems to me you are looking for a compression ignition gasoline engine. Unless the fuel is injected precisely timed like a diesel there is little control on the timing of ignition.
I think the cam profile for this effect would need to permit all exhaust gases to be expelled during the exhaust stroke, recirculating a small amount of exhaust gas REDUCES peak combustion temperatures by effectively lowering the amount of oxygen in the cylinders. I believe Volkswagen pioneered this to overcome local overheating issues in their air cooled engines, it also had the side effect of reducing harmful nitrogen oxide emissions, that is why egr valves are fitted to modern engines.
The cam would also need to allow maximum air charge into the cylinders on the induction stroke. This sets up a conflicting problem, air density decreases as its temperature rises and for the system you have in mind to work the air needs to be very hot. Turbocharging or supercharging might help overcome this as will having a high enough cr to heat cold air sufficiently to ignite the fuel.

In effect you will have built a diesel engine to run on gasoline.
I have no doubt engine manufacturers have experimented with this idea for some time, but none of them have developed a viable engine.
The theory is sound enough, making it a reality is some way off yet.

Roger.

 

[dream on mcss]

So basically the idea is just to "diesel" a gas engine?

 

[RockSS]

The reason why nobody makes a gas engine the runs like a diesel is gas burns to quickly and is not efficient. That being said please try to make one that would be cool. Smokey Yunick was a great man. 8)

 

[custom442]

So basically the idea is just to "diesel" a gas engine?

sort of, but not really. Diesel times the entry of fuel and ignites it with pressurized (high temp) air near TDC in the combustion chamber.

Smokey's idea (which worked) is to run a 4 cycle gas engine with spark until it gets to x temperature (over the point of preignition of a normal engine) then cut off ignition all together. When the intake is closed there cannot be any preignition of incoming fuel only preignition of the air/fuel charge which is already in the chamber and compressing just after entry = firing. There is something of a 'diesel' effect without spark but then it still relies on the combustion of a complete air/gas mixture through pressure/heat. BTW this is ridiculously more efficient than current diesel engines because there is a minute usage of fuel with extremely high AFR.

 

[custom442]

It seems to me you are looking for a compression ignition gasoline engine. Unless the fuel is injected precisely timed like a diesel there is little control on the timing of ignition.
I think the cam profile for this effect would need to permit all exhaust gases to be expelled during the exhaust stroke, recirculating a small amount of exhaust gas REDUCES peak combustion temperatures by effectively lowering the amount of oxygen in the cylinders. I believe Volkswagen pioneered this to overcome local overheating issues in their air cooled engines, it also had the side effect of reducing harmful nitrogen oxide emissions, that is why egr valves are fitted to modern engines.
The cam would also need to allow maximum air charge into the cylinders on the induction stroke. This sets up a conflicting problem, air density decreases as its temperature rises and for the system you have in mind to work the air needs to be very hot. Turbocharging or supercharging might help overcome this as will having a high enough cr to heat cold air sufficiently to ignite the fuel.

In effect you will have built a diesel engine to run on gasoline.
I have no doubt engine manufacturers have experimented with this idea for some time, but none of them have developed a viable engine.
The theory is sound enough, making it a reality is some way off yet.

Roger.

Hey roger, thanks for those tips. But what I'm thinking of is reproducing something that Smokey created at some point. The system preheated fuel to around 200-250 degrees from an engine coolant/water coil around the fuel lines. It used that fuel (a lower pressure will heat easier/not fuel injection) to move into a turbocharger which further heated the fuel to 450+ degrees. By this point the autoignition temperature is very close to being reached. Cylinder pressures will autoignitie/preignite the gasoline on compression stroke with the incoming air/fuel charge without spark. In a normal engine this would blow it up for a few reasons:
#1 with normal stoich AFR around 14.7 the combustion chamber would not be 'hot' enough to sustain a constant preignition episode, it'd be the classic example of run-on for a few seconds or so. #2 Spark could not be timed exactly to the varying points of preignition BTDC with varrying rates of fuel & timing vs rpm/vacuum (without millions of dollars in computers which I promise Smokey didn't use)

So the only way to control the burn is with control over the Air fuel ratio, and the only way to get the burn hot enough to preignite itself indefinetely is to use around 28:1 AFR or something ridiculous. (to gain combustion temps just below the melting point of steel)

I understand the concept behind what he did. What I don't understand during this process -> 1) the engine needs to turn on and warm up thermostat (this is one of the reasons he failed at selling it, it was not 'instant' took a few minutes before it could be driven). 2) the AFR needs to to lean out to the point of detonation where the engine can begin to preignite the fuel (so you're working between 2 devils wanting to blow the piston through the crankcase) 3) the ignition is then cut and AFR is leaned out even more to run extremely high combustion temps which allows the preheated fuel to burn evenly based on intake volumes and balanced cylinder combustion


...somewhere between 2 and 3 are where the cam suggestions come into play. Makes sense to expel all gases to increase combustion temps. Intake charge is not really a problem, it would be pressurized air (from turbo) at around 400-450 deg F. What type of durations would I be looking at? I assume he ran a stock lift or similar, the power and efficiency were made in the AFR not the cam. In theory manifold vacuum should be the same at similar rpm with or without the extra heat of the overall engine/incoming fuel/combustion temps.

 

[rustyroger]

You've got me thinking on this one.
I talked to a friend who flies radio controlled model aircraft, he told me they use a 2 stroke engine that runs without a spark on a special fuel, he couldn't tell me much detail about how it works though. If I can find out more I'll post it.

I wonder how mechanically efficient such an engine would be?, that is hp per lb of fuel used.
Steam engines are not much better than 7% efficient, the best gasoline engines I believe 35%, modern diesels may approach 45% in these terms.
Compressing hot gases to auto-ignition temperature would use some power, I understand running the blower on a top fuel dragster uses something like 4-500 bhp, and turbocharging has its own power restricting issues.
The heat could be a free lunch if it was drawn from the exhaust system, it has occurred to me that utilising the exhaust heat to power a steam turbine might be another way of getting the most energy from the fuel burned.

The earliest engines were very inefficient regardless of if they were steam, gas, diesel or gas turbine. Smokeys concept would probably also be very low efficiency until similar amounts of R&D time and money has been spent as on the aforementioned.
Maybe there is a mechanical engineer on this forum who can do the math to help with this one?.

Roger.

 

[Blake B]

this is one of the few places my input will actually be useful your friend is running a nitro(methane) engine, we use them in rc "gas" classes (buggies, monster trucks ect) its essientially a diesel motor in concept.. they use glow plugs though.. oh and nitro is about $30/gal.

 

[custom442]

the efficiency here would be limited by how lean it's able to run. And that's limited by the materials used, if an all iron engine with forged internals/iron heads, steel rings/head gakset etc it should be able to reach a certain amount of air fuel ratio that brings combustion just under the point of melting the steel, around 2500+ deg F (1400+ deg C). I'm not a physicist or engineer so can't relate that in terms of numbers but I have had some upper level physical chem classes so I'd be able to follow along if anyone can do the math.

Smokey's fiero he used this in was originally a mid 20's mpg car with around 90hp/120tq something like that in a 2.5L, the iron duke isn't a high revving engine. He gained around an average of 50mpg and 250 hp, the hp isn't something amazing (unless you take a look at the AFR's he was running) but the mpg is just ridiculous.

What I was thinking whoever wanted to reproduce an engine like this, it's the best example out there of extreme lean burn. If the engine could be brought down from its normal running no spark/high AFR state to standard stoich AFR with spark like gas engines today, it could run a secondary fuel injection system (which would be off most of the time) that would allow it to really boost up the turbo. People have made 650+ hp on turbo iron dukes. It's tricky enough just to go from spark to no spark, but if this system could be made to come back down to spark and lower combustion temps you're really talking max efficiency in terms of fuel burn (extremely lean cruise/high AFR) and switching down to maximum energy output (lower AFR and boost controller to turn up psi). If somehow a cam was designed to be able to fit both of these scenarios I don't see how it would be difficult to achieve a regular 50+mpg/250 useable hp like smokey's design, and then when engine temps/AFR come down boost turned up around 400+hp (wasting a lot of gas then, but it'd be cool to have that feature).

I've looked into this some and it seems there are some Indian and Asian companies who have designs on ceramic piston engines, there are some ceramic rotary engines out right now I believe. This would allow even higher AFR's and efficiency, but I don't know how viable those engines would be at varying rpms since properties of ceramic aren't conducive to heat cycling of a normal engine. Ceramic would be a cheap way to get extremely hot combustion temps to lean burn more than iron, but iron/steel would be the best for power + mpg efficiency.