FloridaRustang
Active member
I probably shouldn't even posit this here... but it's as good a place as any, since the idea came out of further research for one of my off-the-wall ideas (I've forgotten which one, now).
I think it was sleeve valves I was looking up when I stumbled across the Diesel Two-Stroke. With direct injection, it's much more fuel efficient compared to gasoline two strokes because you put in fuel after the chamber's closed. Not only that, but they have an exhaust valve, rather than port and have to be fed via forced induction (read: super- or turbocharger).
Anywho, I've referenced the Miller Cycle before -- several variations have been tried. Mazda actually produced one. Most common method is to hold the intake valves open into the compression stroke (Mazda did it for 20%), effectively reducing the combustion capacity of the engine (3.0 liter becomes a 2.4 liter) but maintaining the expansion capacity of the whole, thus being more powerful than the 80% yet more efficient than the 100%. Real problem, and why you really can't pull it off with a conventional carb, is reversion in the intake manifold. But the main idea is to extract more usable energy from the expansion instead of literally blowing it out the tailpipe.
Two-Strokes are notorious fuel suckers. Heck, for every two revolutions, they get two intake charges, compared to a four-stroke's one. Of course, they tend to be smaller, too. What you really get is power-to-weight ratio at the expense of lousy fuel efficiency.
But I think we can clean it up with a Diesel Two-Stroke using the Miller Cycle.
Huh?
I looked for anyone trying this and no one is saying a word. I assume no one else has thought of it (and it may be obvious why).
To set up, imagine two cylinders, either on the same crank or on sychronized cranks, so that the pistons are both at TDC or BDC simultaneously. The first cylinder (combustion cylinder) has an intake port towards the bottom, like a normal two-stroke, and an exhaust valve, like a normal diesel two-stroke. However, the passage from the exhaust valve dumps directly to an intake valve on the second cylinder. The second cylinder (expansion cylinder), in addition to the intake valve, has an exhaust port at the bottom and an exhaust valve at the top. An overhead cam operates the valves. A supercharger pushes air through the intake port.
So...
At or near BDC, the intake port is open, the exhaust valve is open, and the supercharger pushes clean air into the cylinder until the port is closed, at which point the exhaust valve also closes.
One cylinder's worth of air is compressed.
At or near TDC, diesel fuel is injected into the cylinder, spontaneously combusting and expanding all that air, pushing the cylinder back down.
Now here's the trick: Go ahead and open the exhaust valve. The charge expands into both cylinders (yeah, the second one might be a smaller bore, dunno yet) until the intake port is opened on the combustion cylinder and the exhaust port is opened on the expansion cylinder.
Now at BDC (and really at every BDC), excess exhaust pressure is relieved throught the exhaust port, and fresh air comes into the intake port, until the ports are both covered.
The exhaust valve on the combustion side and the intake valve on the expansion side are both closed, while the exhaust valve on the expansion side is opened to eliminate pumping losses of the remaining exhaust in the expansion cylinder.
Absolute lunacy, but what do you think?
I think it was sleeve valves I was looking up when I stumbled across the Diesel Two-Stroke. With direct injection, it's much more fuel efficient compared to gasoline two strokes because you put in fuel after the chamber's closed. Not only that, but they have an exhaust valve, rather than port and have to be fed via forced induction (read: super- or turbocharger).
Anywho, I've referenced the Miller Cycle before -- several variations have been tried. Mazda actually produced one. Most common method is to hold the intake valves open into the compression stroke (Mazda did it for 20%), effectively reducing the combustion capacity of the engine (3.0 liter becomes a 2.4 liter) but maintaining the expansion capacity of the whole, thus being more powerful than the 80% yet more efficient than the 100%. Real problem, and why you really can't pull it off with a conventional carb, is reversion in the intake manifold. But the main idea is to extract more usable energy from the expansion instead of literally blowing it out the tailpipe.
Two-Strokes are notorious fuel suckers. Heck, for every two revolutions, they get two intake charges, compared to a four-stroke's one. Of course, they tend to be smaller, too. What you really get is power-to-weight ratio at the expense of lousy fuel efficiency.
But I think we can clean it up with a Diesel Two-Stroke using the Miller Cycle.
Huh?
I looked for anyone trying this and no one is saying a word. I assume no one else has thought of it (and it may be obvious why).
To set up, imagine two cylinders, either on the same crank or on sychronized cranks, so that the pistons are both at TDC or BDC simultaneously. The first cylinder (combustion cylinder) has an intake port towards the bottom, like a normal two-stroke, and an exhaust valve, like a normal diesel two-stroke. However, the passage from the exhaust valve dumps directly to an intake valve on the second cylinder. The second cylinder (expansion cylinder), in addition to the intake valve, has an exhaust port at the bottom and an exhaust valve at the top. An overhead cam operates the valves. A supercharger pushes air through the intake port.
So...
At or near BDC, the intake port is open, the exhaust valve is open, and the supercharger pushes clean air into the cylinder until the port is closed, at which point the exhaust valve also closes.
One cylinder's worth of air is compressed.
At or near TDC, diesel fuel is injected into the cylinder, spontaneously combusting and expanding all that air, pushing the cylinder back down.
Now here's the trick: Go ahead and open the exhaust valve. The charge expands into both cylinders (yeah, the second one might be a smaller bore, dunno yet) until the intake port is opened on the combustion cylinder and the exhaust port is opened on the expansion cylinder.
Now at BDC (and really at every BDC), excess exhaust pressure is relieved throught the exhaust port, and fresh air comes into the intake port, until the ports are both covered.
The exhaust valve on the combustion side and the intake valve on the expansion side are both closed, while the exhaust valve on the expansion side is opened to eliminate pumping losses of the remaining exhaust in the expansion cylinder.
Absolute lunacy, but what do you think?
