In order to fulfil emission legislation and achieve good drivability of combustion-engine-powered vehicles, information about the air charge and feedback about the engine condition is necessary. In current systems, different sensors are used, e.g. the MAP (manifold air pressure) sensor and a lambda sensor.Aiming at reducing costs, efforts are being made to reduce the number of sensors while still retrieving the necessary information. Various engine speed based functions are state-of-the-art for automotive engines, e.g. for fuel-calibration, misfire-detection etc. Those functions evaluate the engine speed fluctuations during a working cycle induced by combustion. For multiple-cylinder engines, those influences are overlapping, therefore evaluation possibilities are limited.The work presented is based on the effect that at a single-cylinder engine, there is no overlap of combustion influences of various cylinders on the crankshaft. Therefore, it is possible to not only evaluate the combustion phase and attribute the result to a certain cylinder, but also to evaluate the other phases of the working cycle. Correlations between features based on engine speed and various measured reference values can now be investigated, especially during compression, but also during the intake- and the exhaust-phase, without the influence of the combustion of other cylinders.This paper presents possibilities of estimating the air charge and the indicated mean effective pressure (imep), based on the engine speed measurements of a single-cylinder engine of a small motorcycle. These measurements are treated with a sophisticated model-based evaluation that takes into account the physical phenomena. Using these models significantly reduces the calibration effort and the necessary number of maps compared to simple engine speed evaluation methods.In this paper it is shown that the necessary information for air charge and imep estimation is contained in the engine speed signal. A very good correlation to values calculated from reference sensors is achieved. The computational effort and the necessary calibration effort are lower than for multiple-cylinder engines, yet the achievable precision is higher.The resulting algorithms are suitable for real-time calculation on modern Bosch MSE (Motronic Small Engines) control units.