An inline 4-cylinder turbocharged direct-injection gasoline engine was used to compare the thermal efficiency of the engine with lean burn and exhaust gas recirculation(EGR)

and the comparative test was carried out at the lowest fuel consumption point of the universal characteristic curves. The experimental results show that both lean burn and EGR can effectively reduce engine fuel consumption. When the dilution rates of lean burn and EGR are 33% and 19%

their maximum brake thermal efficiencies can increase by 2.8% and 1.7%

respectively

and the brake thermal efficiency with lean burn is even up to 39.9%. The main reason for higher thermal efficiency of using lean burn is the reduction of heat transfer loss and unburned fuel loss. From the view of combustion

the dilution effect of lean burn and EGR extends the ignition delay and combustion duration. At the same dilution rate

the ignition delay with lean burn is shorter than with EGR

and the higher dilution rate of lean burn results in less heat transfer loss. However

use of EGR helps to suppress the occurrences of knock and to advance combustion phase

which makes the exhaust energy loss lower than with lean burn. As to exhaust emissions

both lean burn and EGR can obviously reduce NOx emission at high dilution rate. At the same dilution rate

HC and CO emissions with lean burn are lower than with EGR because of rich oxygen concentration

which results in a much lower unburned loss of lean burn.