Dynamics and Control of a Free-Piston Diesel Engine
Tor A. Johansen□1, Olav Egeland□, Erling Aa. Johannessen□□ 2 and Rolf Kvamsdal□□
□ Department of Engineering Cybernetics,
Norwegian University of Science and Technology,
N-7491 Trondheim, Norway.
□□ Kværner ASA, Technology Development
Postboks 169, N-1325 Lysaker, Norway.
Free-piston diesel engines are characterized by freely moving pistons without any crankshaft or camshaft
connected to the pistons. This allows a compact and efficient engine design, but requires automatic control
of the piston motion. This paper present a dynamic mathematical model of a free-piston diesel engine, and a
control oriented dynamic analysis leading to a piston motion control structure. Experimental results using a
full scale test cylinder are included and show feasibility of the suggested control approach.
The free-piston diesel engine concept was developed by Pescara, and engines of various size were manufac-
tured between 1930-1960 by GM, Ford, Renault, Junker, Sigma and others, see (Moiroux 1958, Klotch 1959,
Underwood 1957) and the more recent review (Achten 1994). The basic idea of the free-piston diesel engine
is to avoid the crankshaft in the traditional diesel engine and to use a power turbine instead to convert energy
from the exhaust gas. In addition, the camshaft is replaced by some other mechanism to actuate the valves.
Hence, the pistons move freely in the cylinders, mainly influenced by pressure forces. The main advantage of
the free-piston diesel engine compared to a traditional diesel engine is that there are few moving parts, making
the engine compact and mechanically simple. Furthermore, the two-stroke free piston engine has high thermal
efficiency. This is in particular true at part load where one can take advantage of the additional degrees of free-
dom provided by a freely moving piston and let the positions of the bottom and top dead centers depend on the
load in order to optimize efficiency and emissions. Thus, the free-piston diesel en