BMW is looking into ways to maximise waste heat recovery in vehicles with two pathways being explored – thermoelectric and thermodynamic.
At the SAE 2009 World Congress, it presented an analysis of two basic configurations of the Rankine cycle applied to a thermodynamic heat recovery system in a four cylinder combustion engine. It has found, through bench test measurements that waste heat recovery can provide an additional power output of around 10 per cent.
Honda is also looking into use of a Rankine cycle co-generation unit in an effort to boost efficiency of a hybrid vehicle by capturing waste heat exhaust. It presented test results in 2008 that showed that at 62mph consistent driving speed, the use of the Rankine cycle improved thermal efficiency by 3.8 per cent.
This has been further backed up by research at Loughborough University and the University of Sussex, both of which found that using waste heat from light duty vehicles could deliver fuel economy advantages of anything between 6.3 per cent and 31.7 per cent.
The Rankine cycle brings the working fluid to high pressure, feeds it with heat, generates high energy dense steam and then converts it into mechanical energy. The BMW study used two basic single loop systems – an exhaust gas only as the heat source; and exhaust gas and coolant.
Based on quantitative analysis, it found that water delivered the highest thermal efficiency for system A, whereas ethanol is preferable in system B. BMW found system B shows higher potential at typical highway speeds although this should not be determined as a general recommendation.
To conclude, BMW suggested that there was an increase in engine performance in the range of 10 per cent close to the road resistance curve for the top gear – therefore, there is a remarkable increase in fuel efficiency.
