Virtual Encoder for Achieving Crank Angle Resolution Measurements of In-Cylinder Pressure in Small Engines by using Time Based Data Acquisition

Small size engines feature several peculiarities that render them a challenge with respect to implementing measurements required for characterizing specific phenomena such as combustion evolution. Measuring in-cylinder pressure is well established as standard procedure for determining combustion characteristics, but in the case of small size units actually applying it can require alternative approaches. Fitting a crank angle encoder may be extremely difficult, as a consequence of the actual size of the power unit. Cost is another essential driver for small engine development that also influences how measurements are implemented. Within this context, the present work describes the development and implementation of a method that employs an algorithm that practically generates a ‘virtual’ encoder. Only a basic phasing signal is required, such as an inductive crankshaft position sensor output or that of an ignition pulser. The software was developed on an experimental engine with a crank angle encoder, that provided the reference case. Several configurations were under scrutiny, so as to identify the minimum requirements able to fulfill the intended task. Afterwards, it was tested for achieving crank angle resolution in-cylinder pressure measurements by applying time based data acquisition on up to 8 high speed channels (with a maximum sampling rate equivalent to 0.5 crank angle resolution at 6000 rpm). Measurements showed that the proposed method successfully fulfilled both requirements, i.e. high accuracy and cost effective data acquisition on two small size engines (one single cylinder 50 cc and the other 3 inline cylinders 600 cc). Simulations performed using the 0D/1D approach also confirmed the validity of the results. The only major drawback that was identified at this stage is that the proposed method requires the acquisition of data on one or two additional channels (for crank shaft position/ignition pulser signals) for ensuring correct implementation. Nonetheless, the benefits can be considered as more than sufficient for minimizing the effects of this shortcoming.