A rotary Wankel type steam engine is an ideal power producer for automotive applications. The Wankel is a displacement engine that the steam passes through with no reversal in direction (uniflow) and the exhaust is from max displacement to min. displacement.
This engine is similar to the Wankel I.C engine. With only one rotor we get
two overlapping power cycles per crank rotation. A power cycle starts every 180°
of crank rotation and lasts 270°. That's 90° of overlap, 1/3 of the power cycle.
Exhaust takes another 270° of rotation. A complete cycle is 540° of crank rotation.
The engine is a three-lobe rotor design. As the rotor rotates on the eccentric
crank each lobe defines a variable volume chamber. In the following diagrams we
follow the engine through its cycles, Also note that crank rotates 3 times the rate
of the rotor. I.e. For every 90° of crank rotation the rotor rotates 30°.
In the illustration to the left color is used to illustrate the engine cycles. Bright red is the live throttled steam from the boiler. The darker red is the steam expanding after the inlet valve closes. Steam engines use cutoff, the practice of admitting steam for part of the stroke and expanding over the remaining portion, to control the torque of the engine. A long cutoff is used, and necessary, for starting from a stop or hard pulling. Cutting back on the cutoff gives better economy. Green is used to indicate the exhaust steam leaving the engine. Comparing a single rotor engine with single cylinder double acting piston engine we see the same number of power strokes in a revolution. But in the rotary engine the power cycles overlap 90°. No dead spots!
In a Wankel type engine the effective clearance or compression is function of
the K factor, rotor radius (distance from an apex to the rotor center)/eccentric
(crank throw), and rotor dome. The K factor determines the maximum compression
and rotor lobe must be domed to achieve the minim-desired clearance.
A compounded engine is then constructed of a high pressure and low-pressure rotor. The high-pressure exhaust port directly coupled to the low-pressure intake port during the high-pressure exhaust cycle. The engine is configured such that the low-pressure lobe's power cycle is coincident with high-pressure lobe's exhaust.
In the illustration of the compound Wankel to the right, color is also used illustrate the steam state. A very light blue used for the inlet steam becoming darker and darker as it expands through both rotors until it is finely exhausted. This engine designed with a 3 to 1 volume ratio between low pressure rotor and high pressure rotor achieves a steam expansion of 8.8 with long starting cutoff of 180° and goes to 15.0 under normal cutoff of about 90°
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