Engine with Telescoping Cylinders

by Stan Lass

Telescoping cylinders make large one cylinder and inline two cylinder internal combustion engines viable as transportation engines. The engine thermal efficiency can be improved by using a piston which moves in a telescoping cylinder which moves in a fixed cylinder. This allows operating the engine faster while keeping the friction reasonable and reducing the heat loss to the coolant for each combustion.

Larger cylinders have less friction per horsepower developed, resulting in less efficiency loss due to friction. Much of engine friction comes from the piston and ring friction, which increases as the square of piston speed. By using a piston in a telescoping cylinder to reduce the ring sliding speed, the engine's most efficient rpm can be roughly doubled. (The telescoping cylinder moves at half the piston speed which halves the ring sliding speed for a given piston speed.)

When the engine rpm is doubled, there is a nearly 50% reduction in heat loss to the coolant per combustion. Some of the heat not lost is recovered by a higher bmep (brake mean effective pressure) and (further) overexpansion. The reduced friction (per cubic inch of displacement) of a large cylinder engine allows greater overexpansion before the benefits are negated by increased friction.

An estimate of the improved thermal efficiency is perhaps 3-4% improvement for an engine with one telescoping cylinder, more for an engine with two or more telescoping cylinders. The estimates are based on 12+% of the fuel energy lost to the coolant during combustion and expansion. A single cylinder auto engine might have one telescoping cylinder, a truck engine two and a locomotive engine three.

The telescoping cylinder could include an enclosing jacket through which oil is pumped, thereby cooling the telescoping cylinder. The jacket would be smaller in diameter than the inside of the cylinder in which the telescoping cylinder slides.

Regarding means to move the telescoping cylinders, one approach is to have the piston, it's connecting rod and it's crankpin be conventional. To move the telescoping cylinder at half speed, crankpins at one-half the radius of the piston crankpin radius, with a crankpin on each side of the piston crankpin, and with the same phase as the piston crankpin, be connected via connecting rods to the telescoping cylinder. The telescoping cylinder would slide in guides (which could be extensions of the fixed cylinder) in order to maintain alignment in the fixed cylinder. The guides would also prevent twist of the telescoping cylinder. When the piston is at top dead center, the telescoping cylinder would also be at top dead center.

A second approach is to have two or more levers, spaced around the piston. One end of each lever would be connected to the engine frame. The other end would be connected to the underside of the piston (or wrist pin) via con rods. Then different con rods connected to the bottom of the telescoping cylinder, each connecting down to the midpoint of the corresponding lever. With this arrangement, the telescoping cylinder moves half as far as the piston.

To recap, beginning at piston bottom dead center, the telescoping cylinder top would be a half stroke down from the engine cylinder top while the piston top would be a full stroke down. When the crankshaft is at the top dead center position, the piston top and the telescoping cylinder top are both at the top of the engine cylinder.

A recent BMW two cylinder inline twin has pistons which go up and down in unison. A third crankpin, 180 degrees away from the piston crankpins, drives a weight via a con rod. The weight counterbalances the pistons movement. The following is a more complicated balance mechansim.

The imbalance of a single cylinder engine can be lessened by a mechanism which moves the crankshaft assembly opposite to the direction of the piston movement. With this mechanism, the piston-wristpin-upper connecting rod movement is counterbalanced by the crankshaft assembly movement. The crankshaft assembly includes the crankshaft, the lower end of the connecting rod and the cradle that holds the main crankshaft bearings. The crankshaft would be counterbalanced (in rotation) to include the weight of the lower end of the connecting rod. With a conventional piston cylinder combination, there would still be a secondary imbalance due the connecting rod not being fully balanced. (This imbalance also occurs in a typical 4 cylinder engine.)

The crankshaft assembly would likely be several times heavier than the piston assembly, and so the magnitude of the crankshaft movement would be several times smaller than the piston movement(stroke) to get good engine balance. The crankshaft assembly center of gravity would be roughly in line with the cylinder axis.

The mechanism to move the crankshaft consists of a cradle which holds the crankshaft main bearings. The cradle is hinged to the side of the engine frame so as to allow the crankshaft center of gravity to move mostly opposite to the piston movement, connecting rods connect eccentrics on the crankshaft to anchor points on the engine frame. They have the same phase as the crankpins.

Then as the crankshaft rotates, the eccentrics cause the crankshaft to move (mostly) away or towards the piston. Movement of the crankshaft is otherwise constrained by the cradle. The crankshaft would move opposite to the piston and telescoping cylinder movement. Note that the eccentrics are crankpins with a short throw (radius).

The combustion pressure on the piston is transmitted to the piston connecting rod to the crankshaft, then through the eccentrics to connecting rods to the engine frame. For a vertical cylinder axis, the crankshaft main bearings in the cradle serve to keep the crankshaft correctly positioned, side to side, while the cradle allows the crankshaft to move mostly up and down, as determined by the eccentrics connected to the engine frame by connecting rods.

Note that the piston stroke would shortened by the crankshaft movement, and that shortening can be compensated for by an increase in the piston crankpin radius. If used, the telescoping cylinder crankpins radii would also need to be increased.

Since the crankshaft moves back and forth along the direction of the cylinder axis, some means of coupling to a non-moving shaft would be desirable. Various couplings could be used, e.g. an Oldham coupling, a gear to a mating gear, two universal joints or a double crank four bar linkage. When using a double crank four bar linkage, the radii of the cranks, the offset of the fixed shaft and the phase of the cranks can be chosen so as to increase the crankshaft rpm during combustion and early expansion, thereby lessening the heat loss to the coolant. Alternatively, for a diesel engine, could use the double crank four bar linkage so as to decrease the crankshaft rpm during combustion, allowing for more complete combustion.

Ordinarily, the torque reaction due to compression and expansion of a large cylinder engine would be difficult to cope with. For example, an engine that develops it's power most efficiently at 2400 rpm might be expected to develop useful power at 800 rpm. At 800 rpm the force of the torque reaction lasts 3 times longer than at 2400 rpm. Assuming compliant engine mounts, the torque accelerates the engine 3 times longer, but the engine twist is 3 * 3 times as much. (An analogy is that a ball which falls for 3 seconds falls 3 * 3 times as far as a ball that falls for 1 second.)

What is needed for the above is a mechanism which, aside from engine starting and warmup, runs the engine at say 2400 rpm. Engine - Flywheel Hybrid describes a mechanism which allows operating an engine faster and still be efficient. It does this by controlling the frequency of combustions, with the engine crankshaft stopped between engine combustion cycles.

The engine movement, or twist about the crankshaft axis, due to the torque reaction can be lessened by adding weights at some distance from the crankshaft axis.

For an engine operated twice as fast, the cylinder displacement can be half as large for essentially the same power. A smaller cylinder lessens the force of the torque reaction due to compression and expansion. This would improve improve the power to weight ratio for large cylinder engines.

For larger cylinders, it may be desirable to use a telescoping cylinder within a telescoping cylinder, etc. The strokes of the sliding cylinders could be apportioned such that the ring sliding speed is comparable for all rings, including the piston rings. Note that the piston speed can be much higher than the piston ring sliding speed.

The compression ratio can be varied by replacing the engine frame anchor points by eccentrics mounted on the engine frame which allows the anchor points on the engine frame to be moved up or down. The movable anchor points are connected to eccentrics on the crankshaft by connecting rods. Moving the anchor points changes the average position of the crankshaft, effectively varying the position of the piston and telescoping cylinder at top dead center, thereby achieving variable compression.

An inline two cylinder engine could be built as two one cylinder engines with the same phase on all of the crankshaft crankpins.

At some point, the difficulty of getting adequate volumetric efficiency would likely limit the useful number of telescoping cylinders in an engine cylinder. To improve the volumetric efficiency, individual poppet valves could be setup for dual use, i.e. allow both intake flow and exhaust flow. For these dual use valves, a valve would close off the intake port side during exhaust flow and a valve would close off the exhaust port side during intake flow. A mix of intake only poppets, exhaust only poppets and dual use poppets could be used, depending on the desired flow capacity. Some of the dual use poppets could close late in the exhaust flow, switch over to intake flow by closing the exhaust port side valve and opening the intake side port valve and then the poppet valve could reopen for intake flow. In some cases, it may not be necessary to close the poppet during the switchover. The switchover of the other dual use poppets could be delayed to piston top dead center. The valves to control intake and exhaust port flow could be butterfly valves.

With an Engine - Flywheel Hybrid , could stop the crankshaft for one flywheel revolution to allow added time for a uniflow purge in an otherwise conventional 2 cycle engine with poppet exhaust valves in the head.

The Engine - Flywheel Hybrid works best with one or two cylinder engines. The perceived need for more power per cylinder led to telescoping cylinders. The need for engine balance with one or two cylinder inline engines led to the movable crankshaft. The need for more intake and exhaust flow area for volumetric efficiency with a high piston speed led to the dual use poppet valves. Variable compression facilitates HCCI engine operation.

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