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Sankey diagram:
In the Sankey diagram (also called the “heat balance diagram”) the energy losses of the internal combustion engine can be shown. The Sankey diagram is derived from (and is calculated using) the Seiliger process.
The fuel supplied to the internal combustion engine is mixed with air and ignited. Not all of the combustion energy is used to drive the engine (and the wheels). More than half of the combustion energy is lost through, among other things:
- Drive losses (think of driving the coolant pump, air conditioning compressor, timing drive, and other mechanical components)
- Powertrain losses (converting the translational motion of the piston into the rotational motion of the crankshaft)
- Heat losses (heat from the combustion is lost through radiation and reflection to the engine components)
The useful energy that remains is called the effective efficiency.
The image shows the Sankey diagram. The diagram shows that 35% of the fuel is lost as exhaust gas heat, as well as 30% cooling loss (heat that is transferred to the coolant), 8% mechanical loss (drive and powertrain losses), and 6% radiation.
This diagram is of a petrol engine. A petrol engine has a relatively low efficiency of around 21%. That means that of one litre of fuel, only 21% is used for “propelling the vehicle”. A diesel engine has a higher efficiency (up to 35%). In that case, for example, the cooling losses and radiation are lower, but the mechanical loss is somewhat higher. A diesel engine therefore has a different Sankey diagram than a petrol engine.
To reduce, for example, the cooling losses as an engine designer, one can choose to use a coolant pump with a variable speed (possibly also switchable) or to apply multiple cooling circuits. By using an adapted design, it can be ensured that less coolant flows along the cylinder walls, and therefore less heat is absorbed by the coolant.
By applying an exhaust gas turbocharger, the exhaust gas losses can be reduced. The effective efficiency increases. With a mechanical supercharger, the mechanical loss increases (because an additional component needs to be driven), but the effective efficiency increases more. From the 5% extra mechanical loss, for example, the effective efficiency is increased by 10%.
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