Idea for 2-stroke diesel

Factual Questions
1

I have a concept and I’d like to bounce it off you guys to test the feasibility of it.

First I wish to state two established engineering foundations:

  1. On many 2-stroke diesels, the intake charge of air is introduced via port valves…essentially holes at the bottom of the cylinder which are uncovered by the reciprocating piston…and the exhaust valves are standard poppets found in the traditional location of the cylinder head.

  2. In the V8 found in the Corvette and other high performance GM cars, the technology of Reverse Flow cooling is employed; pressurized coolant is first directed to the water jacket in the cylinder heads, then to the water jacket in the rest of the block. This is to provide cooling to the hottest part of the engine first. This also allows a higher compression ratio for the same octane rated fuel.
    (note: some of the rules which govern gasoline combustion do the opposite results for diesel :confused: )

What if one could combine the two concepts?

Take a turbo and direct the compressed charge to an air jacket , first at the head, then to the rest of the block. As the air reaches the port valves, the coolant air now becomes the intake charge. Since the air is preheated, there is no need to maintain an extremely high compression ratio diesels are known for, thus, a lighter, less stout engine can be used.

Furthermore, parasitic losses from the water pump and radiator fan are eliminated: all cooling comes from the free/cheap energy recovered in the exhaust stream and used by the turbo.

I would think this engine would be very thermodynamically efficient; instead of kinetic energy being used up to get rid of thermal energy, thermal energy is concerved and recycled, such as in a regenerator or recuperator in a gas turbine.

Is enough intake air being used to keep this engine from melting?

2

This may be the sticking point.

Without an intercooler, the air from the turbo is going to be hot already, which will limit the amount of cooling it can do as it flows around the cylinder. You may need ceramic cylinders, or at least some design capable of handling very high temperatures.

An intercooler may help, but then you’re rejecting a lot of heat which runs against the aim of high efficiency.

3

I think that you’d have to come up with some arrangement for starting the engine from cold, as there would not be any source for preheating the air.

If you preheat the air, it will expand, and this will reduce the compression ratio, so you’d need to compress the hot air before introducing it to the combustion chamber. In fact in engines that use pre-compression, its usual to have something to cool the air down before the combustion chamber, hence the term intercooler, and this allows for a more dense air charge.

The only way you are likely to get your air up to a significant enough temperature is probably some form of heat exchanger based around the exhaust manifold.

The big problem with two strokes is that the inlet port and outlet are open at the same time, and rely on back pressures to hold fuel/air in, and this only happens efficiently at certain frequencies and timings, and at most others there is a loss of efficiency, hence the noticeable power band of two strokes, where the engine suddenly reaches an efficient operation speed.

It also means that two strokes tend to spew out plenty of hydrocarbons in various states of combustion, and also, because the crankcase is used to derive precompression, it means that some bearings are swilled with fuel, which does not make for a good lubricant, so special oils are added to the fuel to overcome this, and these oils are then burnt and become nasty emissions - its a total loss lubrication system.

A recent developement that has not yet really caught on, is to have precompression derived, not in the crankcase, but by a pump driven by the rotating crank.
This allows the crank and bottom end etc, to have their own lubrication system, which cleans up emissions no end, and by using direct injection, it can be arranged so that the outlet valve can be closed befor fuel/air mixute is intorduced.

There’s much more to it than that though, because the is a pre mixture stage, where some air is mixed prior to introduction, then injected along with the main air charge, and timing is critical, as always, and has to be very precisely variable to take into account not only the usual burn rates for the fuel air charge, but also to ensure that this fuel air mixture hit the chamber at the right time just after the main air charge is introduced, the idea is that the fuel air charge forms a stratified layer around the spark plug.

Now this works in a petrol engine, but its taken a huge amount of R&D to get it going as a viable engine, and its yet to take off in a big way.

Two stroke diesels are used in very large units, you compress your air first, and as the outlet valve closes, you direct inject fuel under very high pressure, these all have precompression from superchargers, or a turbocharger.

There are a few differant possible configurations, from having inlet and outlet valve, just like a petrol engine, to just outlet valve with an injector inlet - the most common, to a weird sort of thing where the crankcase has two parts, one part, the lower is the bearing housing with its usual supply of lubricant, and a sealed upper gallery that is ported to the pistons so you get th piston compress into the cylinder on the upstroke and compress to the gallery on the downstroke

4

I think there’s a thermodynamic problem with your idea. The share of combustion heat that is taken away by your cooling air will also heat it up and, at the same pressure, will decrease its density. In other words, you only want air heating up when it’s captured by the piston, so you can harvest some of the heat as mechanical motion. And you don’t want to do anything to decrease the density of the air - in fact even though it takes a great deal of energy to pump air into the engine at higher than atmospheric pressure, the resulting density increase is so beneficial it makes it worthwhile to do this “supercharging”. In fact in your engine you are already decreasing the effective compression ratio because you’re lowering the intake air density.

Also, regarding the discussion about 2 stroke designs - there are very simple two stroke deisels, such as model airplane engines, and quite simple lawnmower and motorcycle engines - then there are lots of medium sized engines in cars and trucks and so forth that use 4 strokes - then it’s back to 2 stroke deisels for the largest engines, but with very different means of introducing and removing the gasses. Look around at the designs for the largest marine deisels, on the order of 10,000 to 80,000 hp. These can be made more efficient than 4 stroke engines can. It’s fascinating.

5

Sorry, a bit of a hijack here.

This company’s stock has been hot the last couple days. They have a new diesel engine technology.

http://www.regtech.com/

6

Link to a reply on another board:

http://biodiesel.infopop.cc/eve/ubb.x?a=tpc&s=447609751&f=869605551&m=606104916