This post is a continuation/generalization/more organized version of my earlier blog post.
There are a lot of improvements possible for internal combustion engines (aka ICE’s). It helps to list the areas that are causing losses, to start:
– The geometry of the physical layout of the piston, connecting rod and the crankshaft is less than ideal. The connecting rod needs to be ~60 degrees past top dead center to get the best leverage on the crankpin; but the pressure from the fuel ignition occurs much earlier than this; when the connecting rod is essentially trying to bend the crankshaft sideways. The motion of the piston is necessarily sinusoidal.
- The power stroke is only 25% of the full cycle, and there is a lot of mass that has to be accelerated, stopped and accelerated again.
- The valvetrain has to physically resist being moved, and it has to work against the air flows.
- The piston tends to scrape the sides of the cylinder, because it would “rather” twist that stay straight. The rings must exert friction on the cylinder.
- The oil must be pumped through little tiny passageways.
- Electricity must be generated.
- An ICE is a self-powered air pump, in essence. Air flow and the pressures generated, and the cyclical nature of them cause resonances, and backpressures, and the gasses become spring-like.
- Small volumes, like the space above the top ring and the top edge of the piston, trap unburned fuel because the flame cannot reach it.
- Everything flexes and springs — the crankshaft and the camshaft flex torsionally and longitudinally, the piston vibrates and distorts, as do the cylinders. Valves bounce and stretch and distort into potato chip shapes.
The list goes on… The net result is a typical internal combustion engine that uses ~20% of the energy in the fuel for output motion at best, and requires a transmission to keep the torque of the engine relatively close to the speed of the vehicle.
So, knowing all this, how can we make incremental or wholesale improvements?
+ Offsetting the crankshaft center away from the power downstroke gives the connecting rod some better mechanical leverage — but is the compression stroke adversely affected?
+ Variable valve timing allows the torque to be available over a broader range of RPM’s.
+ Valves can be electrically/hydraulically moved in both directions (opened and closed) to avoid fighting the springs. This also makes it easier to use subtle or more abrupt adjustments to the valve timing.
+ Use cams rather than the crankshaft, to gain a lot more mechanical leverage, and to allow the piston motion to be controlled by the designer; like the Revetec:
This particular design also reduces piston scrape (but it introduces some tendency to rotate the piston within the cylinder). It also avoid big changes in crankcase pressures (in configurations with even numbers of pistons). This design effectively doubles the efficiency.
+ Use the Atkinson valve timing, like the Prius does, which has a lot of overlap of the exhaust valve with the beginning of the intake downstroke (I think?) so that there is built in exhaust gas recirculation (aka EGR). This also effectively doubles the efficiency.
Hmmm, how well would a 2-cylinder Revetec with Atkinson cycle and electrically activated valves work?
+ Use a rotary design that reduces the reciprocal motion.
+ Use a 2-stroke design to cut the parasitic losses in half.
++ Use a continuous burn design to further reduce the cyclical nature of the engine; or at least reduce the time between power cycles.
+ Figure out how to reduce waste heat from being produced, and then try to use the remaining excess heat to produce output.
What are other ideas to improve ICE’s?
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While power plant efficiency is a very important factor to the overall vehicle’s efficiency, there are lots of ways to improve the rolling chassis, as well.
Rolling efficiency is the most basic function of any vehicle; however it may be powered. This involves:
* Tires, wheels, wheel bearings, suspension, wheel alignment (loaded and in motion).
* Ride height and attitude — both of these are critical to good aerodynamic drag, and we should not leave them to chance.
* All aspects of aerodynamics: overall shape and size, specific details, ventilation of the passenger compartment, motor/drivetrain cooling/temperature control. By using good passive air management, we can both improve the air flow around and through the vehicle; and avoid needing a power input to actively solve these requirements.
* Weight and friction of all moving parts (if you can avoid power steering and power brakes, this reduces the losses of operating the vehicle).
+ Temperature stability affects a lot of things: the people, and the drivetrain in particular. Learning from buildings, we should use insulation and low-e glazing to help stabilize the temperatures.
+ Braking should be regenerative: either electrical whenever possible, or, we should use hydraulic motors and a small accumulator; instead of friction brakes which produce waste heat.
+ Especially if the brakes are regenerative hydraulic, then the suspension should also be regenerative; and use the shock pistons to also pressurize the accumulator. If possible, the entire suspension springing should be hydraulic, I think. Because flexing springs also produce waste heat. Alternatively, the suspension could be electromagnetic.
Can you add to this list of improvements, please?
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{ 1 comment }
The bike tips the scales at just 231 lbs (105 kg) and features an efficient, fuel injected, four stroke 125cc single, producing 9 hp (6.85 kW).
Because the modifed bike now required less energy to go down the road than the stock version, its gear ratios could be optimized: engine RPM could be lowered for a given speed that would have lugged the engine before the aero mods. Reducing engine speed normally improves fuel economy.
