Engine Design Project

[gordon_dunn]

Hiceadha - Sounds like you wouldn't go far wrong if you had 'ptwizz' and 'pete_b' on your design team- these guys in collaboration could produce some very interesting things....

2 strokes are fascinating- simple in theory, but in reality an art or a science?

[ptwizz]

To arrange a 2 stroke engine that doesn't burn oil requires that the air does not go through the crankcase (with the exception of experimental dry ceramic bearing engines).

In most cases, this means driving a supercharger.

The key to getting power from the engine is volumetric efficiency. The action of the expansion pipe can increase the mass of fresh air in the cylinder at the moment the ports close. Ideally, the returning pressure pulse from the pipe not only pushes air back into the cylinder, but also some exhaust. In carburetted engines, this reduces or eliminates unburnt fuel exiting through the exhaust.

In a direct injection engine, the mass of fuel injected is still limited by the mass of air in the cylinder.

In both cases, when the engine is operating in its peak range, the result is the same.

Direct injection comes into its own when the engine is used for much of the time at below peak output. At lower RPM and gas flow rate, the expansion pipe returns its (weaker) pressure pulse substantially before the exhaust ports close. This allows some of the fresh air to flow out of the cylinder and into the exhaust. In a carburetted engine, this air is already mixed with fuel, so some fuel is wasted.

The YPVS valve was intended to adjust the timing of the exhaust port to take advantage of the expansion pipe over a wider RPM range.

I have some ideas of my own on the subject of cleaner, more effecient 2 stroke design. These ideas involve a hypocycloidal crank and double acting piston(s). I already have too many projects in my workshop, but I would be happy to discuss with anyone who fancies building a prototype.

[PatPux]

I was ok till .........Hypocycloidal!

[Guest]

Tut,

Don't you know anything ! lol

SW

[ptwizz]

You're on the internet!... where do you think I get my information from?

[Hiceadha]

Would a 2-stoke engine with one of those injection conversion kits still have the oil mixed with the fuel? would this not be less reliable... It seems far more realistic to design and build a basic fuel injected 2-stroke then a direct injection although im guessing the benefits in emissions and economy won't be nearly as good

[ptwizz]

A direct injection 2 stroke requires a seperate system to distribute oil to the bearings. If the engine has crankcase induction (conventional) then it either burns that oil or it needs a means to keep the oil out of the induction air. This is possible, using a complex array of seals at each bearing and a dual path oil system.

[pete_b]

The amount of oil that then gets burnt is very much smaller than normal hence the low emissions.

[ptwizz]

True, Pete.

If the oil is delivered directly to the bearings and cylinder wall, the total rate of oil consumption is reduced.

Why is this not done in carburetted 2 strokes (to my knowledge)? The modern 2 stroke engines I have dealt with have a metered pump, but it just squirts the oil into the cylinder and crankcase to thrash around with the fuel/air mix in the accepted manner.

[ScoobySnacks]

PT, I think that you have there one of the classic trade-offs that plague all engineering activities.

Whilst the "pump it into the crank case and let it find its own way into the compressed space, lubricating as it goes" method may seem less efficient, and entail the burning of a lot of lubricating oil, the alternative method (as happens in four strokes, with a separate fuelling and lubricating system) necessitates the inclusion of some kind of sump, a certain volume (and weight) of oil and added bulk in the form of a sump (whether wet or dry). This adds complexity, and possibly more importantly, weight to the system.

If you are going to use a 2 stroke, with a separate oil circulation system that would be required for it to utilise a DI system, then you might as well go for a four stroke. At least with a four stroke you can fit a turbo to compensate for pressure effects with altitude (yes, you can supercharge a two stroke, but it requires a separate power source and again, added weight and complexity).

The big benefit of a 2 stroke is lower weight, higher power to weight ratio and a relatively simple design. The trade off for this is a lower efficiency level and high emissions, both due to the lubricating oil and the large amount of unburnt or incompletely burnt fuel. A four stroke will provide, when properly set up, a much more complete burn of fuel and therefore a cleaner and far more efficient burn. Again the trade off is increased weight and lower P/W ratio, and far more complexity.

It would be interesting to see if you could scale up a model glow plug engine to do the job of a petrol engine. You might then be able to reduce the complexity (no ignition system required) even further, thus reducing the weight more. Of course, being a "diesel cycle" engine, relying on compression to initiate ignition, these engines tend to be prone to vibration, and the head and cylinder would need to be thicker to deal with the higher pressures involved.

The ultimate, for me, would be gas turbine engines. This would get rid of the vibration effects, would give a far improved P/W ratio and would be amazingly cool. Of course, you'd not be able to hear much short of a nuclear explosion over the scream of them......

[Hiceadha]

But in a fuel injection 2-stroke engine where the fuel is not directly sprayed into the combustion chamber then I suppose the oil could still be mixed with the fuel yes? however this would not proof as effiecnt as DI but still better then a carb

[ScoobySnacks]

I'm pretty sure the benefit would be limited, as you are still relying on the fuel splashing up from the crank case, so it doesn't really offer a great deal of improvement in terms of accurately controlling the fuelling.

With the standard 2 stroke design, you need to make sure plenty of fuel is around the place, to distribute the lubricating oil. If you limit the amount of fuel going in to try and get close to the stochiometric ratio then you might well risk lubricant starvation of the bearings.

[Hiceadha]

OK since you guys seem to know a fair bit about engines what do ye think of a 6-stroke small engine for similar use. The 5th stroke being an intake and expansion of air/water then then the 6th a hot air/ steam exhaust stroke. This would cool down the combustion chamber allowing for far better efficiency.

[ptwizz]

1) Gas Turbine: Apparently they don't scale down with the power to weight ratio that might be expected, probably because various ancillary systems (ignition, oil, fuel pump etc.) are very similar for 50kg or 500kg thrust.

http://www.gasturbine.pwp.blueyonder.co.uk/ct3201.htm

2) Sixstroke: The sixstroke concept is nice in theory for improving efficiency, but has some issues attached.

Specific power output is less than either a 4 stroke or a 2 stroke, due to less frequent firing. This is to some extent offset by the additional power extracted on the 6th stroke.

The engine will need a substantial water reservoir, which will be heavy.

http://en.wikipedia.org/wiki/Six-stroke_engine

[Hiceadha]

The six stroke using air for the 5th stroke no water would be ideal then for say lawnmowers or cement mixers in theory then really wouldnt they?

[ScoobySnacks]

With the 6 stroke, I'd be worried about thermally shocking the cylinder and head and causing differential expansion/shrinkage and head damage. Also the rapid thermal cycling could cause damage to the inside of the cylinder and the piston head.

[ptwizz]

Thermal shock shouldn't be too much of an issue. All the heat that goes into the cylinder and head is generated in the few milliseconds of combustion.

The flame temperature (2000°C+) is very much higher than the cylinder & head temperature (100 to 120°C).

Water or air injected at ambient (say 20°C) is only 80 to 100°C cooler than the hardware, so the thermal shock will be much les than during combustion.

[ScoobySnacks]

Good point, well made. I was thinking back to time spent flying 152's, where you had to keep some gas on to stop the engine over cooling and cracking the heads. That was because the heads got pretty warm though, so if the hardware is cooled on each stroke then it becomes less of a problem.

Do paramotors suffer from carb icing?

[ptwizz]

Any engine can suffer from carb icing.

http://www.kfackler.com/gtuf/carb_ice.html

I had a motorcycle which would ice up on icy, damp mornings. Icing would occur at the pilot jet, causing the engine to die at low throttle settings. Not ideal in icy conditions. This was fixed with the addition of electric carb heaters, controlled by a temperature sensor in the airbox.

[ScoobySnacks]

Any engine "can" suffer from carb icing (even those without carbs in extremis) I was more wondering is it something that is experienced regularly by Paramotor pilots?

I was just thinking, if you are cooling the engine block down with the fifth and 6th strokes (ok, preventing them from getting hot would be a better description) then might you increase the risk of icing by lowering the ambient temperature around the engine.

Agreed the carb shouldn't be sitting in hot air from the engine as it will decrease efficiency, but there must be some heat transfer to the carb from the engine. Even if it's only a couple of degrees it might help in marginal cases?

Anyway, that's some impressive thread drift!

[ptwizz]

If the 6-stroke engine were properly designed, the hardware would operate at the same temperature as the 4-stroke. The 5th and 6th strokes recover energy from the exhaust gas.

The temperature at which the hardware operates is limited by the properties of the materials. If it is possible to configure the engine to run hotter, effeciency and specific power output are increased.

Some of the smoothest flying is during the winter months. If you've waited for the fog to clear before launching, humidity will be high. On launching, the throttle will be open for the first 30 seconds or so of flight. All the ingredients for carb icing are there.

I think the thread has managed to stick to the heading "Engine Design Project" fairly well. I haven't seen any pictures of cats, nor any comments on politics, religion etc.

Please sign my petiton to call for a ban on scientologists creating images of gay buddhist felines.

[s1buell_wl]

Please sign my petiton to call for a ban on scientologists creating images of gay buddhist felines.

I didn't see the thread or I would have signed.

[Guest]

Hummmmmmmmm

[youtubevideo]

[/youtubevideo]

SW

[ptwizz]

Not very new.. Google 'sliding vane rotary steam engine' and you'll find a rich history.

Fundamentally, the design has the chamber pressure acting at 90° to the vane motion, which produces the highest possible friction load between vane and housing. Piston engines and Wankel rotaries have the sliding element acting in line with the load, minimising friction load.

Pretty much every possible configuration of engine was invented 100 years ago. One or two may become viable as new materials become available, but the piston and crank will be hard to beat for simplicity and efficiency.