Engine - Flywheel Hybrid

by Stan Lass

A typical car of a few years ago had an overall efficiency of 15-17%, perhaps 18-20% at the engine flywheel. In contrast, the same engine typically has 30% maximum engine efficiency at the flywheel. If a practical way could be found that allowed operating the engine at it's near maximum efficiency as used in a car, then that should net a better than 50% improvement in miles per gallon.

One way is a gas-electric hybrid, e.g. a Prius, which operates the engine at it's near maximum efficiency. The power not needed for powering the car is dumped into a battery for later usage. However, there is an energy loss going into and out of a battery.

Another way is to use a rotary stop start mechanism which allows operating a one or two cylinder engine at part load, yet with the engine's near maximum efficiency. When an engine is operating at it's most efficient rpm, say around 2400 rpm, nobody thinks twice about each piston stopping and starting 4800 times per minute. The rotary stop start mechanism allows selectively operating the crankshaft, a revolution at a time, to develop as much power as is desired. Only the crankshaft is stopped and started, the flywheel goes on spinning and delivering power. The mechanism, including a clutch, fits between the flywheel and the crankshaft.

In more detail, as the flywheel slows, the crankshaft would be engaged, a single compression - combustion - expansion revolution would speedup the flywheel, and then the crankshaft would be stopped a small way into a compression stroke. (For a one cylinder 4 cycle engine, there'd also be an exhaust and intake revolution.) Power taken off the flywheel slows the flywheel. For an automobile, the flywheel would weigh ~10-20 pounds and run at the engine's operating speed. At higher power, the engine would run continuously.

More than one or two cylinders could be used, but they would need to fire within a narrow crankshaft angle.

Definition: A double crank four bar linkage consists of two shafts, each with a crank at the end. The cranks are supported on one end only, like most handcranks. The two cranks face each other. The first crank's crankpin is connected by a link to the second crank's crankpin. The two crank shafts are substantially parallel, but can be offset from each other.

The observation that led to the rotary stop start mechanism follows. Given a double crank four bar linkage, with crank radii(throws) and link of the same length, and a constant angular rate on the input shaft, as the offset between the crank shafts is slowly increased, the angular rate of the output shaft will approach infinitely fast during one part of a revolution and infinitely slow(stopped) during another part of the revolution. For the purpose of the rotary stop start mechanism, the infinitely slow angular rate(stopped) is useful, but the infinitely fast angular rate is not.

To stop the output shaft, the offset is increased as much as is needed to achieve the infinitely slow angular rate(stopped). Away from the output shaft stopped position, the offset is decreased to avoid an extremely high output shaft angular rate. The mechanism to vary the offset can be an eccentric on the input shaft with a link to the engine frame. The input shaft bearings would be mounted on a hinge, allowing the eccentric to move the input shaft. This will vary the crank shafts offset cyclically. With the correct amplitude of motion and the correct phase, this mechanism stops the output shaft once per revolution.

The output shaft will be stopped once per revolution. At the stopped point, the engine crankshaft can remain stopped by declutching it from the output shaft. And to restart, a clutch can engage when the output shaft is once again stopped.

The output shaft minimum offset from the input shaft can be chosen to give a maximum speedup ratio of 1.2-1.8 times faster than the input shaft. Then with the phase between the engine crankshaft and the output shaft chosen such that the output shaft maximum angular rate occurs near the engine's piston(s) top dead center, the time of highest heat transfer is shortened. The average piston speed is the same as if the crankshaft was directly coupled to the flywheel. The peak piston speed is slightly higher. By mostly avoiding an increase in piston friction while decreasing the heat loss during combustion and early expansion, the thermal efficiency of the engine is increased by perhaps 2-3%.

A way to implement the clutch is to split the clutch function into an acceleration jaw which can accelerate the engine crankshaft and a deceleration jaw which can decelerate the engine crankshaft. When both jaws are engaged, the output shaft is firmly coupled to the engine crankshaft.

To stop the engine crankshaft, while the crankshaft is being slowed to a stop by the deceleration jaw on the output shaft, the acceleration jaw is disengaged. During the slowing the acceleration jaw will not be transmitting any force and would be easy to disengage. When the output shaft is stopped, the engine crankshaft stopped. At this point, the engine crankshaft would be prevented from angularly drifting, e.g. by a stop dog or a brake. As the output shaft accelerates away, the deceleration jaw is disenagaged.

To restart, when the output shaft is slowing to a stop, the acceleration jaw is engaged. The acceleration jaw makes contact at the angle where the output shaft was stopped earlier. The stop dog or brake is released. Then as the crankshaft is accelerated out of the stopped position by the engaged acceleration jaw on the output shaft, the deceleration jaw is engaged. The output shaft is then firmly coupled to the engine crankshaft.

With this clutch, the timing of the acceleration and deceleration clutch jaws engagement/disengagement is not critical. There should be minimal shock loading at the point of engagement because the shaft speeds are matched, albeit briefly.

An alternative mechanism for stopping and starting the crankshaft is to increase the offset of the double crank four bar linkage, and hold it there, such that the output shaft's crankpin and the input shaft are coincident. This allows the input shaft crankpin to spin the connecting link around the output shaft crankpin while the engine crankshaft remains stationary. The engine crankshaft must be prevented from angularly drifting. No clutch is needed.

An engine would normally be operated at it's most efficient speed and power per combustion, which is also near the maximum power per combustion. For more power, the engine could be run at a higher speed and with the minimum offset on the double crank four bar linkage, (instead of the varying offset).

To avoid moving the flywheel as the input shaft moves, a second double crank four bar linkage could connect the fixed flywheel shaft to the input shaft. The second double crank four bar linkage could use larger crank radii(throws).

Regarding HCCI(homogenous charge compression ignition) operation, a mechanical compression ratio of say 14-15:1 could be used. As I understand it, achieving consistent HCCI operation is considerably simplified by operating the engine in a narrow speed range and a relatively high power per combustion (for HCCI). The efficiency would be closer to that of a diesel than to a gasoline engine. Emissions are reduced with HCCI operation. There are engines that combine conventional spark ignition and HCCI operation. Atkinson(Miller) cycle would be used to limit the compression when operating in spark ignition mode. (This can be implemented with early or late closing of the intake valves.)

Aside from braking energy recovery, this invention should result in a drive train that is more efficient, lighter and cheaper than a gas-electric hybrid, e.g. a Prius. It avoids:

For an engine generator application, the varying flywheel speed would not be a problem if the flywheel drives an alternator whose output is rectified to DC and then the DC is used directly or is inverted to AC. This could drive electric motors which could drive the wheels of a vehicle.

When the mechanism is used with a conventional transmission, the varying flywheel speed, due to flywheel slowing between combustions, is somewhat of a problem. A heavier flywheel would lessen the speed variation. A slipping wet clutch could eliminate the varying speed into a conventional transmission. Aside from a conventional start in first gear, using part throttle spark ignition operation of the engine for smoothness of power, e.g. 800 to 2200 rpm, the engine flywheel could run in the range of 2200-3300 rpm during normal driving, and also during idling. Some compliance in the drive train would lessen the need for clutch slipping, probably eliminating it during steady speed driving.

A current model BMW motorcycle boxer twin cylinder engine produces ~110 horsepower, from ~72 cubic inches. Two of the preceding could be operated in tandem, giving ~220 horsepower. For more power, a second engine on the other side of the flywheel could provide additional power as needed. The transmission would be driven by a chain, belt or gears from the flywheel. Also, an engine or two could be used on a second axle of the vehicle.

I've done computer simulations. Patent pending.

Some history is that circa 1990 I realized that the efficiency of automotive spark ignition internal combustion engines could be improved by controlling the frequency of combustions. For efficiency, each combustion should be tuned for maximum efficiency. This implies that each combustion should have maximum power (before tricks like running with a richer mixture). If one slows an engine too much, cooling of the combusted charge would lower the efficiency of the engine. Blowby, noise and vibration in a slow running engine could be a problem.

So, the engine, when running, would need to operate at a speed comparable to the speed of a similar size engine when it is most efficient. This defines the problem. Next, is how to accomplish the starting and stopping of the engine with acceptable efficiency.

The path to the rotary stop start mechanism started with cam engines where a cam operates the pistons. These didn't seem promising.

Next, putting a cam on the crankshaft such that when a spring was engaged and the flywheel disengaged from the crankshaft, the kinetic energy of the spinning crankshaft goes into the spring. If one times the disengagement of the crankshaft just right, there'll be enough kinetic energy in the crankshaft to maximally compress the spring just as the crankshaft stops. A brake on the crankshaft would fine tune the stopping point of the crankshaft.(If one disengages the flywheel too soon, the crankshaft will stop, and then the spring force acting through the cam follower will tend to make the crankshaft run backwards.) From the stopping point at the top of the cam, a nudge could be used to start the crankshaft again. This scheme, while tricky, should have worked.

An analogy to the preceding is that of a car coasting to a stop at the crest of a hill. If one starts coasting too soon, the car won't make it to the crest of the hill. If one starts coasting with just the right timing, the car will coast to a stop at the crest of the hill. Then, a little nudge is enough to start the car down the other side of the hill.

A disadvantage of the preceding is that in compressing the spring, one only gets about 95% of the energy back out of the spring. This reduces the overall engine efficiency. A partial solution is placing a double crank four bar linkage, fixed offset, between the crankshaft clutch and the flywheel. With suitable phasing, the flywheel can drive the crankshaft fastest close to when combustion occurs and then when the crankshaft is at it's slowest, the remaining kinetic energy can be stored in the spring. With an rpm variation of two, fastest to slowest, 75% of the kinetic energy from slowing the crankshaft goes into the flywheel. This is an improvement.

In considering how to store more of the crankshaft kinetic energy into the flywheel, an increased offset would do that, but too much offset and it tends to whip the crankshaft too fast, causing unacceptably high stresses. This is when I looked at varying the offset such that the speedup of the crankshaft is limited by lessening the offset at the time of fastest speedup. Simulations of varying offset showed that one could put all of the crankshaft kinetic energy into the flywheel. The rest is covered earlier in this web page.

Also, see Telescoping Cylinders as the Way to Greater Engine Efficiency .

For an animation of the mechanism, see.

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