To improve the actual performance of an automotive engine, an approach consisting of dynamic signal measurements coupled with gas dynamics–thermodynamics process simulations is proposed; this is used to detect the working processes of an automotive engine from cycle to cycle in transient conditions. Based on the working principles and the mathematical models of the proposed detection method, corresponding software was developed, and the reliability of this approach was validated on an automotive engine. Automotive road tests were conducted, and various transient parameters of the engine were successfully detected from cycle to cycle by using the developed software. On this basis, the variations in the influencing factors of and the interactions between various engine parameters were analysed. The results showed that the ignition timing has a strong effect on the indicated thermal efficiency. Where there is an unnecessary delay in the ignition timing, there is a decrease in the indicated thermal efficiency. The pumping mean effective pressure is approximately equal to the difference between the exhaust gas pressure and the intake gas pressure for a low to medium load, but it is much higher than the pressure difference and undergoes great fluctuations in the high-speed and high-load operating regions. Both the presented approach and the research results are significant for improving the engine performance in transient conditions.