|
Perhaps some of you petrol heads could tell me why there are no serious developments with 2 strokes. I know the barrier in the past was possibly horrendous exhaust emissions, but I would have thought that with modern fuel injection systems & computerised engine management, these problems could be resolved!! I believe I read that Ford were running an experimental engine in a Fiesta, but don`t know what came of this. Any info would be appreciated
|
Still gives horrendous emissions;Ford dropped the Orbital years ago because it had trouble meeting the regulations at that time and current and future ones are a small fraction of what they were then-the economy was nothing special either.
It was dropped well before the Ka was even thought of.
|
Dude, this has been discussed before. I have paraphrased one of my earlier postings to answer your question ...
Ford were at one time contemplating the Orbital two-stroke engine which has been under development for a couple of decades now. There are several probable reasons why two-strokes haven't been taken up, including the emission problem mentioned by jc.
One reason for non-take-up could be the cost of complete production line re-tooling. This costs many millions of pounds and would not be viable unless the new engine was sure to be absolutely reliable and more highly desired by the public than what has gone before. A manufacturer would be foolish to make a huge investment without a reasonable chance of a good return. Then there are the technical problems with two-strokes ...
For a two-stroke to meet current emissions legislation it would need to be radically different from the ones we are familiar with. For a start, having piston rings traverse ports in a cylinder wall sets up minute fluctuations of the rings which is known to increase undesirable emissions, so intake and exhaust control needs to be via valves as in a four-stroke engine.
Also, to meet modern economy and emission expectations, the inlet gas must be kept separate from the exhaust gas within the cylinder. Some limited mixing may be useful but this must be very precisely controlled, like the exhaust gas recirculation (EGR) system in some four stroke engines. One patented idea was to keep the gases separated by a disc which rides up and down between the piston and the top of the cylinder under the control of a camshaft system. This was too cumbersome, complicated and expensive.
Another consideration is the heat output of a two-stroke which is likely to be similar to the equivalent power four-stroke, therefore the cooling system is likely to be as large. Further, the two-stroke will not produce twice the power of a similar capacity four-stroke; those in the past were not a great deal over 1.1/2 times the power and it will take a lot of innovative thinking to improve on this without pricing the two-stroke beyond what the market will stand.
So we end up with a two-stroke that is a bit smaller than an equivalent four-stroke but is likely to be just as complicated, relatively expensive, possibly less refined, and with unproven appeal to the new car buyer. That, I would suggest, is why Ford and others are keeping to four-strokes and I can't see that ever changing.
If we do change, this will not be to the two-stroke engine as we know it but will be to variable-stroke engines which, by virtue of variable valve and injection timing/events, will be able to change between two-stroke, four-stroke and possibly even six-stroke operation whilst on the move. For example, four-stroke would be the norm, with two-stroking for short high-power bursts and six-stroking for economical light-load cruising.
We may also see more semi-random skip-firing where the engine cuts out a cylinder every so often, normally when running light. Semi-random operation is fairly easily achieved via the ECU and has already been seen in some fields, such as military engines. The type of engine heard in the distance can be recognised by its acoustic signal, i.e. the sound pattern of the exhaust, and skip-firing can make this harder to detect which would give an advantage on a battlefield. Totally random skip-firing is not so good, especially for diesels, as one or more cylinders could repeatedly miss a firing and hence cool down too much for efficient firing when they are needed again.
I think that's enough for now! Hope I've made my point!
|
|
Not bad but you've missed one important point-crankshaft lubrication-you can use petroil or pressure feed to the bearings but in both these cases you have to dispose of the oil-yes,more hydrocarbons.There was a Californian firm trying to develop an engine with a conventional crankcase and a supercharger to feed air into the cylinders but the sealing between top and bottom of engine is the problem-not heard anything about them for a long time.The Orbital used a seperate feed to the bearings and had a little tank that had to be filled with oil regularly and had a warning light to tell you when to fill it and a limit switch to shut the engine down before it ran out of oil.
|
Interesting point, jc. However, valved two-stroke engines can use a conventional sump and oil distribution system, exactly as a four-stroke. There are still lots of problems though, like the fact that the small end bearing in a pressure-charged two-stroke engine is under constant load which makes it difficult to get a good oil film between the bush and the gudgeon pin.
A Frenchman called Melchior is/was recognised as the two-stroke design expert. He developed answers to a whole host of two-stroke problems and his designs included pressure-charging, variable valve timing, spherical small ends (with an oil pumping action) and much else. He spent a fortune on patents aided, I think, by the French government, but I don't believe any of his engines became commercial realities.
|
|
"Uniflo" system(valved 2stroke)-also used on steam engines.On 4-strokes the next big advance will probably be getting rid of camshafts and operating the valves by solenoids-you then have total control over amount of opening,timing or not opening at all.
|
Yes, it is possible that we'll see the end of camshafts at some stage - that's what I was referring to when I mentioned variable valve timing. Unitary valve operation has been investigated by most engine companies over the past ten years or more but there are still problems to be overcome.
Unitary valve operation can be via electrical solenoids or hydraulic feed but, whatever is used, space has to be found for it. With a camshaft, a closing valve is actually helping to turn the camshaft and therefore reducing the engine power needed to open another valve further along, but you don't get this benefit with solenoid operation so the solenoid is going to be quite large and you will have four of them per cylinder in a four-valve engine if you want full control of the valves.
There is also the problem of accelerating a solenoid-powered valve in a controlled manner, which is what you get with the ramp on the camshaft lobe. This is especially important as the valve closes because you don't want it hammering into the seat. Some designs have included a cushioning spring but this has the disadvantage that the valve can be held by the spring in the part-open position on failure of the solenoid or its electrical supply and this could cause the valve to be hit by the piston. This and much else has to be resolved before we will see unitary valve operation on car engines.
Returning to my previous posting, I should have mentioned that Melchior was involved mainly with *diesel* two-strokes, not petrol. Either way, ports in the cylinder walls are out of the question these days for emissions reasons, though it is possible that some of the very large two-stroke marine diesels still use this system.
|
There is also the problem of *accelerating* a solenoid-powered valve in a controlled manner, which is what you get with the ramp on the camshaft lobe.
>>
And *decelerating* of course.
|
|
Just like to thank Dizzy & ic for their excellent replies, -It never ceases to amaze me the depth of knowledge from respondents to this website.
|
|
Dr.Ehrlich was the petrol 2-stroke man.
|
|
|
