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One of the ways to improve thermodynamic efficiency of Spark Ignition engines is by the optimisation of valve timing and lift and compression ratio. The throttleless engine and the Miller cycle engine are proven concepts for efficiency improvements of such engines. This paper reports on an engine with variable valve timing (VVT) and variable compression ratio (VCR) in order to fulfill such an enhancement of efficiency. Engine load is controlled by the valve opening period (enabling throttleless operation and Miller cycle), while the variable compression ratio keeps the efficiency high throughout all speed and load conditions. A computer model is used to simulate such an engine and evaluate its improvement potential, while a single cylinder engine demonstrates these results. The same base engine was run on the test bench under the Diesel cycle, Otto cycle and Miller cycle conditions, enabling direct thermodynamic comparisons under a wide variety of conditions of speed and load.

Spark ignition engines have low efficiency when working at part and light loads. This can be opposed by using a slightly different thermodynamic cycle known as Miller cycle. This cycle can be implemented in conventional engines if they are equipped with systems providing variable valve timing and variable compression ratio. A computer model to simulate internal combustion engines is used to generate and characterize in terms of torque and brake specific fuel consumption, both Otto cycle engines and Miller cycles engines with VCR. These engines are used in a second computer model to generate the European driving cycle, using the same car and the same gear box, in order to evaluate its performance in terms of fuel consumption. Different ratios for gear boxes using the Miller cycle are proposed in order to achieve a more efficient car (engine, gear box, structure), with less fuel consumption and less green house gas emissions for the urban driving operation.