A Free Piston Engine Generator

A free-piston engine is a linear, 'crankless' internal combustion engine, in which the piston motion is not controlled by a crankshaft but determined by the interaction of forces from the combustion chamber gases, a rebound device (e.g., a piston in a closed cylinder) and a load device (e.g. a gas compressor or a linear aternator).

Free-piston linear generators that eliminate a heavy crankshaft with electrical coils in the piston and cylinder walls are being investigated by multiple research groups for use in hybrid electric vehicles as range extenders. The first free piston generator was patented in 1959.  Examples include the Stelzer engine and the Free Piston Power Pack manufactured by Pempek Systems based on a German patent.  An opposed piston free-piston linear generator was demonstrated in 2013 at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR).

These engines are mainly of the dual piston type, giving a compact unit with high power-to-weight ratio. A challenge with this design is to find an electric motor with sufficiently low weight. Control challenges in the form of high cycle-to-cycle variations were reported for dual piston engines.

Toyota’s Free Piston Engine Generator

In June 2014 Toyota announced a prototype Free Piston Engine Linear Generator (FPEG). As the piston is forced downward during its power stroke it passes through windings in the cylinder to generate a burst of three-phase AC electricity. The piston generates electricity on both strokes, reducing piston dead losses. The generator operates on a two-stroke cycle, using hydraulically activated exhaust poppet valves, gasoline direct injection and electronically operated valves. The engine is easily modified to operate under various fuels including hydrogen, natural gas, ethanol, gasoline and diesel. A two-cylinder FPEG is inherently balanced.

Toyota claims a thermal-efficiency rating of 42% in continuous use, greatly exceeding today's average of 25-30%. Toyota demonstrated a 24 inch long by 2.5 inch in diameter unit producing 15 hp (greater than 11kW).

Features

The operational characteristics of free-piston engines differ from those of conventional, crankshaft engines. The main difference is due to the piston motion not being restricted by a crankshaft in the free-piston engine, leading to the potentially valuable feature of variable compression ratio. This does, however, also present a control challenge, since the position of the dead centres must be accurately controlled in order to ensure fuel ignition and efficient combustion, and to avoid excessive in-cylinder pressures or, worse, the piston hitting the cylinder head.

Advantages

Potential advantages of the free-piston concept include

• Simple design with few moving parts, giving a compact engine with low maintenance costs and reduced frictional losses.

• The operational flexibility through the variable compression ratio allows operation optimisation for all operating conditions and multi-fuel operation. The free-piston engine is further well suited for homogenerous charge comprtession ignition (HCCI) operation.

• High piston speed around top dead centre (TDC) and a fast power stroke expansion enhances fuel-air mixing and reduces the time available for heat transfer losses and the formation of temperature-dependent emissions such as nitrogen oxides (NOx).

Challenges

The main challenge for the free-piston engine is engine control, which can only be said to be fully solved for single piston hydraulic free-piston engines. Issues such as the influence of cycle-to-cycle variations in the combustion process and engine performance during transient operation in dual piston engines are topics that need further investigation. Crankshaft engines can connect traditional accessories such as alternator, oil pump, fuel pump, cooling system, starter etc.

Rotational movement to spin conventional automobile engine accessories such as alternators, air conditioner compressors, power steering pumps, and anti-pollution devices could be captured from a turbine situated in the exhaust stream.

http://en.wikipedia.org/wiki/Free-piston_engine