Few things to improve fuel efficiency:
- Choose Diesel, HCCI or GDCI instead of Otto cycle (the usual petrol engine).
- Take advantage of modern variable valve timing systems, also EIVC or LIVC.
However, if those options are not available, but the engine is an old time spark ignited, throttle controlled engine with a low compression ratio, fixed cam and a carburetor, probably the fuel efficiency is not very good.
The usual way of controlling a petrol engine is such that the engine speed is kept with the transmission at the approximately optimal range, while the produced torque will be controlled by throttle.
The throttle is a key component, because it causes pumping losses.
These are essential:
- The fuel efficiency of a petrol engine is best at full throttle, because pumping losses are minimized.
- The fixed valve timing with a cam is always a compromise.
- The fuel consumption of an engine is minimized at low engine speed.
It would be ideal if the petrol engine could be run always full throttle and as low speed as possible. Then the fuel efficiency would be constantly maximized.
What is needed:
- A transmission that allows low engine speed. That means very tall gears.
- A petrol engine that runs and produces high torque in a wide range from very low speed to high speed.
There are all kinds of automatic and manual transmissions, also infinitely variable. Anyway, every vehicle needs a complete transmission from the engine to tires, but finding a suitable components may be difficult after all. Tall gears would be needed, but also conventional short gears, so the complete transmission should have a wide range of gears from short to tall.
Choose the main transmission components:
- A gearbox (manual or automatic) with a proportionally wide range of gears.
- A suitably tall final drive gearing.
Certainly the modern variable valve timing systems and four valves per cylinder together will lead to generally improved fuel efficiency. But if the engine is an old two valves per cylinder and no hope about getting variable valve timing, what could be done?
If the engine should be optimized for low speed, it could be done with different valve timing, but that would also make the engine much worse at high speed, reducing performance and power. Probably that would not be acceptable on the long run. Swapping to a smaller engine would be more convenient option.
There is an unusual but real possibility to use such intake valves that are almost like conventional poppet valves but they include a reed valve. The valve will not really open before the pressure differene allows it. The hot exhaust gas from the cylinder will never flow to the intake manifold. These valves may be used in conventional piston engines, but there is a drawback of greater moving weight, which limits the useful speed range of the engine. Anyway these may greatly extend the low-end torque of a petrol engine, which is the desired effect.
Two patents describe that kind of valve:
- US 6,659,059 Reggie Dwayne Huff: "Variable displacement valve seat for internal combustion engines"
- US 6,598,577 Vincent R. Marino: "Intake valve"
Although a solution for valve timing is necessary, it is not sufficient. When a petrol engine runs at very low speed, the gas flow is slow and turbulence may not be adequate. The gas flow may be almost laminar, making the mixture of air and fuel slowly burning. The slow burning rate of fuel would be acceptable if it did not also mean increased risk of detonation, which is a destructive phenomenon. Thus, the flame propagation should always be fast enough to avoid detonation. Practically this means that something should ensure proper fuel mix before ignition.
There is a method to increase turbulence inside the combustion chamber just before ignition. These are called turbulence grooves. Obviously these concentrate the squish into narrow and high velocity streams that generate the necessary turbulence.
There is also a patent that describes the turbulence grooves:
- US 6,237,579 Somender Singh: "Design to improve turbulence in combustion chambers"
It is unfortunate that those inventions are not very good in modern OHC and OHV engines. Due to their greater weight the special valves would fit better to engines that would never run at high speed. Also the turbulence grooves would fit better to engines with greater squish areas above the pistons. On the other hand, the fortunate thing is that such engines exist that fulfill both requirements. Side-valve engines are already poor breathing and they produce their peak power at low speed. Also the squish areas are typically very large.