Vehicles use fuel because the energy in the fuel goes to these uses:
- Air resistance. Because vehicles aren't moving in a vacuum, the air has a resistance force and because force times distance is energy, this uses energy.
- Rolling resistance. Although tires eliminate friction, there is little rolling resistance that converts kinetic energy to heat when moving.
- Acceleration. Getting a mass up to speed requires energy. When braking, this energy could in theory be stored somewhere, but in practice this is rare and when done (e.g. in hybrid vehicles), the efficiency is low.
- Uphill resistance. When moving uphill, the car has to overcome gravity. Sure, when moving back downhill, the energy stored in the gravitational field is released, but then it is generally lost to braking.
- Engine losses. Engines lose energy to the cooling system and to the exhaust system.
- Accessory losses. You have electrical loads in the car such as lights. Furthermore, power steering, engine cooling using the water pump and air conditioning use energy.
The only way to improve vehicle fuel economy is reducing the amount of waste energy going to one of these uses. If somebody claims to have a fuel economy improving solution but fails to tell which category of these it belongs to, it is almost certainly not a genuine solution.
There are at least these technologies that improve the fuel efficiency of the vehicle:
- Mass reduction. Lower mass means lower uphill resistance and lower acceleration resistance.
- Frontal area reduction by making the car e.g. lower. This reduces the air resistance.
- Better aerodynamics. Having a certain shape for the body reduces air resistance for constant frontal area. Probably the best practical body is the Kammback body.
- Higher tyre pressures. Having more pressure means lower rolling resistance.
- Radial tyres that are used in all vehicles today instead of older bias ply tyres.
- Engine downsizing. This is used often in conjunction with technologies that squeeze more power out of small engines by improving the torque or redline RPM so that acceleration does not suffer
- Turbocharging. Turbocharging recovers some of the energy of the exhaust to compress more air to the engine.
- Intercooling. This prevents engine knocking problems in turbocharging.
- Exhaust gas recirculation. This reduces throttling losses and heat rejection due to the lower temperatures.
- Higher compression ratios. According to basic thermodynamics, compression ratio tells what the maximum theoretical efficiency of an engine is. Unfortunately, high compression ratios can cause knocking.
- Compression ignition, i.e. diesel engines. This allows using very high compression ratios and lean burn and removes throttling losses entirely.
- Electrically powered accessories such as power steering, air conditioning and electric water pump. When the accessories are powered electrically, they don't need to spin at a speed proportional to engine speed, and therefore, efficiency is improved.
- Electronic fuel injection. By precisely allowing control of the air/fuel ratio, fuel economy is improved. Electronic fuel injection also allows engine braking fuel shut off.
- Lean burn. When using stoichiometric air/fuel-ratio, there is always a minor amount of unburnt fuel. Lean burn eliminates this unburnt fuel almost completely.
- Emulated Atkinson cycle. In a conventional Otto cycle engine, during the end of the expansion stroke, the pressure in the cylinder is greater than air pressure, and therefore, when the exhaust valve is opened, some useful energy is lost immediately. Emulated Atkinson cycle uses valve timing that pushes back some air-fuel mixture into the intake manifold at the beginning of the compression stroke. Then the compression ratio is lower than the expansion ratio, and therefore, the cylinder pressure is similar to air pressure when opening the exhaust valve.
- Variable valve timing. This allows the engine to be optimized for all RPMs instead of only certain RPM.
- Direct injection. This technology allows ultra lean burn.
- Common rail injection in diesel engines. The higher pressure allows better fuel atomisation.
- Cylinder deactivation. Deactivating cylinders means there are less cylinders drawing air through the intake manifold, reducing pumping losses.
- Offset crankshaft. The forces are the greatest during the expansion stroke and if the crankshaft is not offset, there are lateral forces. An offset crankshaft eliminates these lateral forces and thus reduces friction.
- Lower viscosity lubricants. The viscosity of lubricants means the lubricant does not flow freely and therefore requires energy to flow. Having engines that work with lower viscosity lubricants mean that these lubricant flow losses are reduced.
- Increased number of gear ratios. This means the engine is more often operating at the optimal RPM, and thus fuel efficiency is improved. This technology can be taken to the extreme with a continuously variable transmission (CVT).
- Start/stop system. This system stops the engine when standstill, eliminating idling losses.
- Regenerative braking. This kind of system uses a generator that stores energy into a battery as a brake. This is best done in a hybrid car, but also non-hybrid vehicles can have some regenerative braking if the charging system voltage is kept low when cruising and suddenly increased when engine braking.
- Locking torque converter. Torque converters have some slip, and therefore, they lose energy as heat. By mechanically locking the torque converter at high speeds, the energy loss is eliminated. This improves efficiency of automatic transmission cars.
- Hybrid technology. This technology uses electricity as another power source. The result is that engine efficiency can be improved by engine downsizing or using Atkinson cycle without reducing acceleration.
- Variable valve lift. This reduces pumping losses by using valve lift instead of throttle body to control the air intake into the engine.
- Multivalve technology. By having more than two valves per cylinder, the combined area of all valves can be improved. This is almost always implemented by a dual overhead camshaft.
Most of the techniques cannot easily be applied to existing cars, and thus, the best way to improve fuel economy apart from driving economically is to keep it in mind when purchasing the next vehicle. If a fuel economy improving solution is not in this list, it is likely the solution does not really improve fuel economy (although it can be the case that I forgot something important from the list).