Abstract
The aim is here to evaluate the difference between the energy and exergy (or available energy) balances when heat recovery is considered in an internal combustion engine. In the first case, the entropy of the system is not taken into account so that, the maximum useful work recoverable from a system can not be estimated. Then, the second case is much more adapted to estimate heat recovery potential. In this paper, two modern engines are evaluated. First, an up-to-date gasoline engine: three-cylinder, downsized, low friction, then a modern common rail downsized Diesel engine. For each one, two energy and exergy balances are given for two different part-load operating points representative of the NEDC cycle using experimental data from steady state engine test benches. For the Diesel engine, it is shown that effective work represents around 30% and that around 55% of the energy introduced into the combustion chamber is lost (in the form of heat), especially in exhaust gas, in water coolant and oil. But when considering exergy balance, only 12% of the total exergy introduced through the fuel can be recovered, in order to produce useful work. Expecting a 25% exergy recovery efficiency, the effective engine efficiency could be increased by 10%. For the gasoline engine, the increase of the output work could be around 15%.
Highlights
Thanks to continuous researches and developments on light and heavy duty engines, their efficiency are still improving but are today limited at best to around 40%
As the energy contained in the fuel is calculated with the Lower Heating Value (LHV) and the exergy contained in the fuel is calculated with the Higher Heating Value (HHV), in this paper, the ratio HHV/LHV is taken equal to 1.053 according to Szargut and Styrylska’s studies [2]
Experimentations coupled with relatively simple calculations enable to achieve relevant energy and exergy balances for both up-to-date Diesel and gasoline engines
Summary
Thanks to continuous researches and developments on light and heavy duty engines, their efficiency are still improving but are today limited at best to around 40%. A potential way to improve the efficiency is to recover a part of the thermal energy available in exhaust gas or in coolant. To evaluate the impact of such a new system, one of the first step is to evaluate energy balances in up-to-date IC engines (both gasoline and Diesel). It is important to keep in mind that, energy in IC engine may be available either in the form of work or in the form of heat. When heat recovery is considered, a new conversion of the recovered energy into useful work needs to be done, decreasing the overall efficiency of the energy recovery. To achieve relevant energy balances, taking into account heat recovery, the difference between the energy and exergy (or available energy) has to be noticed. Two energy and exergy balances are given for two different part-load operating points representative of the NEDC cycle using experimental data from steady state engine test benches
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