An internal combustion engine (ICE) transforms the heat energy of fuel into mechanical energy during combustion with the oxygen in the air. Combustion produces burned gas, the pressure of which sets the pistons in rectilinear motion, then transformed into crankshaft rotation movement.
Each of these transformations involves a loss of energy, the value of which much depends on the engine charge and speed. Using a gearbox is therefore essential in order to adapt the charge and thus optimize the output of the engine. The engine + the gearbox form the powertrain and it is the output from this unit that is taken into account.
Andre Douaud standard diagram “Automobiles and Energy” P.32
Improving output obviously means reducing the various losses from intake to exhaust, including the combustion chamber and not forgetting the crankshaft, water and oil pumps and gearbox, etc...
The combustion output, relative to the transformation of energy from the fuel into work by the pressure of gas on the piston, is close to 50% whether the engine has spark ignition (typically for petrol engines) or compression ignition (diesel). The corresponding loss of energy can be found in the heating of the cylinder walls and the hot exhaust gases. The combustion output is somewhat better if that combustion is fast, complete and takes place at a precise moment in relation to the dead center of the piston.
Friction between the pistons and cylinders also constitutes a major source of losses: around 50% of the mechanical energy received from the gas pressure. In this respect, the quality of the lubricant is essential, particularly in terms of control over its viscosity at low and high temperatures, its resistance to the pressure of the segment along the walls of the cylinder and its chemical stability in the presence of unburned fuel, etc...
Losses due to oil viscosity are reduced much faster when the engine heats up quickly, hence the use of a dual cooling circuit and a thermovalve.
Combustion engine performance has constantly improved from the outset. For example, the output per liter of displacement – cc - (kW/l) for a diesel engine was multiplied by a factor of several hundred between 1897 and 2013 to reach more than 150 kW/l.
Starting back in the 1980s, petrol and diesel engines were both to undergo a veritable revolution in their fuel and air supply systems.
N: Speed (rpm)
The spark ignition engine
The spark ignition engine (petrol, gas, ethanol) requires the preparation of an air-fuel mix in stoichiometric proportions before it is introduced into the cylinder. Combustion is then triggered by the spark from the spark plug. The quantity of mixture to be introduced into the cylinder is regulated by the position of the throttle valve and taken in by the piston during intake. The throttle valve plays an important role in restricting intake, especially when the engine operates at partial charge. This causes losses by the pumping of air specific to this type of engine.
Combustion produces burned gases that activate the piston. The very high temperature of the gases causes heat losses in the cylinder walls and exhaust line. Partly cooled as the mixture is taken in, cylinder walls are mainly cooled by the circulation of cooling liquid up to the radiator (by the circulation of air in air-cooled engines).
The spark ignition engine has seen extensive progress, with the carburettor being abandoned in favor of injection of the stoichiometric mixture of air and petrol into the injection pipe of each cylinder (direct injection) and then directly into each cylinder (direct injection). The combustion output can be further improved thanks to stratified injection which enables the exchange of heat to be reduced by keeping the mixture on the periphery lean when the engine is running at low charge, which enables the intake valve to be kept wide open.
Injectors have become high-tech mechanisms that need to resist the high temperatures in the combustion chamber, deliver extremely precise flows in a short space of time. This is why direct injection calls for pressure in the region of 100 bars, versus a few dozen bars for indirect injection.
Another choice consists of regulating the air mass taken in by continually varying the openings of the intake valves which thus enable the throttle valve to be eliminated. The difficulty was to manage to optimize the periods of time and the raising of valves according to the engine speed. This system can also be mounted on the exhaust camshaft thus enabling all distribution parameters to be controlled.
Electronics have enabled extensive progress to be achieved, including electronic ignition and the control of injectors that enables triggering of the spark to be finely adjusted together with injection of the mixture in relation to the upper dead center according to the engine speed. Electronic control in a loop of the pinking associated with the increase in the octane index of fuel has enabled a significant gain in combustion output to be achieved by practically eliminating auto-ignition.
Doubling the number of intake and exhaust valves (transition to 16 V for 4 cylinder engines) enabled a notable gain to be achieved in cylinder fill and empty rates and better distribution of the mixture in the combustion chamber during intake.
Use of the engine at maximum output, i.e. in the maximum output zone, is only effective in a few exceptional cases. Use of a turbocompressor associated with a reduction in the engine cubic capacity enables output to be improved at partial charge, that is to say in 90% of cases. New engines are smaller (downsizing), do not constantly have their maximum output available but come with a system able to give extra power instantly: supercharging with a turbocompressor as well as hybridization with an electric motor or a kinetic energy recovery system (KERS). The turbocompressor can either use part of the energy of the burned gas or be powered electrically. For a turbo powered by burned gas, an air intake cooling system can be used in addition to obtain even greater efficiency. The electric turbo, as it functions without any link to the engine speed, enables a dynamic response to be obtained at low engine speeds.
Another way consists of deactivating one of the cylinders in stabilized operation and reactivating it when extra power is required. The charge of a smaller cc engine is therefore improved but friction is still the same. Some engines are now built directly with three cylinders in place of four or four cylinders in place of six, while delivering the same power.
The diesel engine
The compression ignition engine (diesel engine) uses a fundamentally different combustion process. The quantity of air is determined by natural aspiration or the supercharging pressure. The absence of a throttle valve allows full air intake irrespective of the charge and the air pumping losses observed in the spark ignition engine do not exist in a diesel engine. The fuel is then injected into the pressurized air and combustion is spontaneous due to auto-ignition. The engine torque is regulated by fuel flow control. Thus auto-ignition, causing pinking, to be avoided in spark ignition engines, is at the core of operation here. However, auto-ignition must be controlled by a rigorous fuel supply strategy. The appearance of the turbocompressor triggered by exhaust gases enabled an additional quantity of air to be forced and the cylinder filled at a differential pressure in the region of 1 to 2 bars. Then the direct injection of the fuel into the combustion chamber and transition to 16 valves enabled a new gain to be achieved before the veritable revolution occurred with the advent of the common rail in 1997. The fuel is pressurized to a level over 1000 bars, thus fostering instant vaporization in the cylinder. The electronic control, precise to within a tenth of a millisecond, and new submicronic precision injectors together with optimization of the shape of the combustion chamber have enabled substantial diesel gains to be achieved. New improvements came in the mid-2000s with multi-jet injection or stratified injection: up to 5 injections per cycle enabling better performance of the engine when cold and also reducing speed, vibration and pollutive emission. The latest progress involves cooling the air from the turbo to further increase the intake quantity.
This quantity of air, a maximum whatever the charge (quantity of fuel injected) enables a diesel engine to have a better output than a spark ignition engine, at partial charge in particular. In fact, since the excess air does not participate in combustion and is therefore at a temperature below 1000°C, it is located along the walls, thus limiting heat loss.
Finally, the absence of pinking by diesel engines authorizes higher compression rates than those of gasoline engines (in the region of 16 for direct injection diesel engines versus 10 to 12 for gasoline engines). This compression rate is chosen according to other engine parameters in order to optimize output.
The higher combustion temperature of diesel engines has a major disadvantage in that its production of nitrogen oxide is higher than that of spark ignition engines.
Looking to ideal combustion: uniform combustion
Combustion output is much better when combustion is fast and takes place near the upper dead center. A new process at the demonstration stage in the mid-2000s is aimed at combining the homogeneous nature of the air-fuel mixture of the gasoline engine and the auto-ignition by compression of the diesel engine. These are “Controlled Auto-Ignition - CAI“ and “Homogeneous Charge Controlled Ignition – HCCI“ engines.
In this way, auto-ignition of the air-gasoline mixture is no longer avoided but used. The difficulty resides in very accurate control of the process.
Combustion speed is no longer obtained by a high temperature which results in a near-complete reduction in the production of nitrogen oxides and burned particles and by a significant reduction in heat loss on cylinder walls.
Neither gasoline nor diesel has been optimized for this type of engine and synthetic fuels are being studied, based on various fossil or biomass fuels.
Adapting the engine to the vehicle power requirement: the gearbox
The torque that can be supplied by an engine depends on its rotation speed (engine speed) and its charge (the quantity of fuel burned per cycle). The output (in watts - W) is equal to the product of the torque (in Newton meters - Nm) and the angular speed of rotation (in radians per second - rd/s).
The torque curve which characterizes any engine, gives the value of the torque according to the rotation speed. This curve is obtained at full charge, that is to say, when the accelerator is down to the floor.
Andre Douaud book standard diagram “Automobiles and Energy” P.36
Maximum output is not necessarily obtained at maximum torque, it all depends on the type of engine and the vehicle it is fitted into. Irrespective of the type of engine, the torque curve, giving the torque according to the engine speed, involves a maximum zone and deteriorates at a varying pace at lower or higher engine speeds. The same applies to the output, particularly for spark ignition engines at low speed due to losses from pumping. The role of the gearbox is to adapt the engine speed to the vehicle speed in order to operate in the maximum charge zone.
This constant search to improve energy efficiency has thus led to an increase in the number of ratios: transition from 3 to 6 gears on passenger vehicles from the 1960s to the 2000s. At the same time, dual-clutch robotized gearboxes made their appearance following on from simple automatic gearboxes. They function in automatic or sequential (manual) mode with several tens of millisecond gains in time when changing gear, thus avoiding uncomfortable jerks and, above all, a loss of power. Numerous electronic assistance devices are available to the driver, fostering increasingly efficient optimization of output.
Andre Douaud book standard diagram “Automobiles and Energy” P.39
Possible improvements for all types of engine
The reduction in losses linked to engine friction, the optimization and fine-tuning of water pumps and electric water pumps, the recovery of residual heat, direct injection, a reduction in cubic capacity with turbocompression, the deactivation of cylinders and new systems controlling distribution or the compression rate all contribute to mechanical efficiency.
Electronics enable us to envisage variable valve timing (VVT), variable control for valves and their electromagnetic activation, that is to say camless. This flexibility associated with the variability of compression rates opens up avenues for new improvements in output and the use of auto-adaptive fuel “Flex-fuels”.
Improvement in energy efficiency is no longer limited to the powertrain alone: Hybridization enables it to operate in its maximum output zones. For example, this is the case for stop and start systems, recuperative braking (or regenerative braking) and more complete electric or hydraulic hybrid technology or even inertial technology for certain buses with specifically equipped stops in cities. The advanced stop and start technology enables the engine to be stopped while the vehicle is at a standstill, then economically restarted, thus avoiding consumption when idling.
The improvement of combustion engines has not reached its limits. Downsizing with turbo, the development of homogeneous combustion and combustion using new types of fuel, control and command electronics in addition to or to replace certain mechanical parts and hybridization, electrical in particular, show there is enormous potential for progress. What is the limit? What will it cost? The impact on the environment is likely to play a major role in these upgrades.
Examples that can be developed in detailed data sheets:
- Valeo Stop & Start
- Fiat multi-air
- Ford 1.0 eco-boost
- BMW Valvetronic