Design of an ECU for a free piston engine

Abstract

The recent trend towards sustainable growth has shed more light on the way energy is used and how the planet is being affected by uncontrolled emissions. The transport sector and its almost total reliance on liquid fossil fuels is a major polluter. Until recently, free piston engine technology has been viewed as a technological dead end. However, it now appears that these have a rather niche application to fill. Tough regulations, especially relating to emissions, have accelerated the evolution of the internal combustion engine. Free piston engines lack both a crankshaft as well as a camshaft, making them rather different in comparison to conventional internal combustion engines. Free piston variants, previously thought to be nothing more than research instruments are now emerging as a promising short to medium term solution to the range of problems encountered by pure electric and to some extent, hybrid vehicles. The Achilles' heel of the current breed of electric vehicles is their range, as their reliance on charging points, especially those that can rapidly replenish their batteries, make them unattractive to customers. An interesting configuration for the free piston engine, involves coupling it with a linear alternator, effectively converting the engine's mechanical power to an electric charge. When working in tandem with an electric motor, the free piston engine can effectively be considered as an auxiliary power unit, supplying energy to the electric motor but never involved into the resulting energy conversion from electricity to motion. Previous projects have tackled various mechanical deficiencies of the engine. Unfortunately, dynamic control was severely lacking. The main aim of this project, therefore, was to design and implement an electronic control system that could be improved and refined, thereby paving the way for the linear alternator concept. The electronic control unit was based around an embedded system, featuring a development board, a mesh of sensors which supplied data related to important variables in real time and the actuators themselves ( spark plugs and injectors ). By analysing the inputs, iii the engine management code was programmed to continually monitor the timing parameters, hence adjusting the actuators according to lookup tables. In addition, a graphical user interface function, controlled through the engine management system, dynamically displays the important variables including piston position, piston frequency of oscillation, throttle position, coolant temperature and engine load via the MAP sensor. A key feature of the system is its modularity, both in terms of hardware and software. Future expansion for research purposes was catered for in two ways. Modification of the lookup tables is possible through any text editor and is independent of the engine management software. In addition, the code is written in clearly defined modules so any future development, or changes, to the logic of the management system, can be attempted without undue difficulty.