Specific fuel consumption:
We usually express the fuel consumption of a vehicle in the number of kilometres driven per litre, e.g. 1:15. In the vehicle documentation, the value in litres per 100 km is often given. Here, the driving conditions are taken into account, i.e. the driving resistances, which play an important role.
For technicians it is interesting to know how much fuel it costs to deliver a certain power over a period of time. This consumption is expressed in kilograms of fuel per hour (B). When we look at it per kilowatt, we refer to the specific fuel consumption (be), expressed in g/kWh.
The specific fuel consumption can be included in the torque-power diagram of the vehicle. This diagram shows that the specific fuel consumption under full-load conditions is at its lowest at the moment when the engine torque has just passed its maximum.
Engine efficiency:
We obtain the lowest specific fuel consumption under the conditions in which the engine efficiency is the highest. Power is expressed in
(kilo)Watt or Joule per second. The input power is the heat content of the fuel, which is equal to the specific fuel consumption (be) * the delivered power (P) * the specific heat of combustion (H).
In the example below, the efficiency is calculated from the diagram where the specific fuel consumption can be looked up.
Example: 110 kW engine.
- The lowest fuel consumption under full-load condition is obtained here at an engine speed of approximately 2000 rpm and amounts to 200 g/kWh
- The delivered power according to the graph is 65 kW at 2000 rpm, so the output power is equal to 65,000 J/s.
- The minimum consumption is 200 grams per kW per hour.
- Converted to consumption in grams per second, this becomes:
- The input power was the heat content of the supplied fuel. For regular petrol, the lower heating value is taken as 42.7 MJ per kg and for diesel fuel 45 MJ per kg
- 0.00361 kg of fuel per second releases the following amount of energy as heat when burned:
- The efficiency is then:
- The efficiency of 40% applies at that moment at which the delivered power and fuel consumption apply. The efficiency is different at other engine speeds and when the accelerator pedal is not fully depressed, so that the power shown in the diagram is not being delivered.
Power diagram / BSFC map:
In the test phase of every (new) engine, a measurement of the specific fuel consumption is carried out. In this measurement, the fuel consumption is measured on an engine test bench or dynamometer at different engine speeds and variable engine loads. The load is adjusted by pressing the accelerator pedal gradually further down, so that the engine delivers a few kW more power at each step. In this way, the entire speed range is covered.
The figure below shows the fuel consumption diagram, also called the “BSFC map” (egg diagram). The islands indicate the fuel consumption in g/kWh. These egg-shaped lines connect the points where the specific fuel consumption is equal. The smallest island indicates, at a speed of around 3000 rpm, the lowest fuel consumption, namely 240 g/kWh. We call this the “sweet spot”. At such an engine speed and load, the engine is at its most economical.
Explanation of the lines in the BSFC (egg) diagram:
- Vertical axis: the torque in Nm;
- Horizontal axis: crankshaft speed;
- Blue line: the torque curve of the engine;
- Green lines: power lines in kW;
- Black islands: the consumption areas
In the (green) power lines it is clearly visible that at a decreasing engine speed, the torque (and thus the mean effective combustion pressure) must become greater in order to maintain the same power. At the same time, we also see a decrease in fuel consumption. The minimum fuel consumption of 240 grams per kWh is obtained at a speed of around 3000 rpm with a delivered power of approximately 85 kW. The fuel consumption of this car is on average 9 l/100 km.
This means that the engine is most economical when it has to deliver about 45% of the total power. At lower power outputs the engine is inefficient: hardly any power is being delivered, but all internal friction losses still need to be overcome. In practice, this can mean that the vehicle may be more economical when driving at 120 km/h in 6th gear than when driving at 90 km/h in 4th gear.
Power diagram with downsizing:
Until recently, manufacturers used engines with a large displacement. For example, in the VAG group the 6.0 (W-) 12-cylinder engine was the showpiece in the Audi A8, and the BMW M5 (E60) offered high performance with its naturally aspirated 5-litre V10 engine. The mid-range models were also equipped with a relatively large displacement, e.g. a naturally aspirated 2.0-litre. Nowadays, manufacturers look for every possible way to drastically reduce emissions without compromising performance. We see the displacement of more and more engines becoming smaller, with an exhaust gas turbocharger providing good performance. An example of this can be seen in the VW Golf, where the 1.0-litre engine with turbo performs better and is more economical than an (older) 1.4-litre engine without turbo:
- VW Golf V from 2005, displacement: 1.4 litre, power: 59 kW, consumption: 6.9 l/100 km (1:14.5);
- VW Golf VII from 2015, displacement: 1.0 litre, power: 85 kW, consumption: 4.5 l/100 km (1:22.2).
The BSFC maps below are of a naturally aspirated engine with a displacement of 2.5 litres and a boosted 1.6-litre engine. Both engines deliver a maximum torque of 240 Nm. The torque curve of the naturally aspirated engine is much flatter than that of the turbo engine around 3000 rpm. With both engines, the maximum torque is reached at approximately 3000 rpm, but we see that the brake mean effective pressure (BMEP) of the turbo engine is 7 bar higher at the torque speed. A higher BMEP leads to less flow losses during gas exchange and a higher efficiency.
