Quasiturbine> Product> Intake Cut Off Valve

 


It is good to have a basically efficient motor / expander,
but to get the most of it, it must be used in an efficient manner.

Intake Cut Off Valve
Single Stage Efficiency Versus Power

Note: This is not necessary for the motor to run great!


Quasiturbine Efficiency

The Quasiturbine is quite efficient as is by many standards. But for those who want still more efficiency, the intake cutoff is a possibility for the compressible fluid modes (air / gas / steam motor). For all Single Stage Positive Displacement Expander (including piston) without cut-off valve, theoretical efficiency (not comparative efficiency to SSE Model) is 71 % at 1 bar, but fall under 42% at 7 bar, while maintening the advantage of full nominal motor power output. Vane expander are not doing as good due to chamber load losses (see theory section below).


Power Level versus Efficiency

First, great care must always be taken not to confuse motor efficiency with the system efficiency. A 100% motor efficiency could still led to a low system efficiency, as efficiency of each system component multiply themselves... Efficiency is not a constant motor parameter, but it depends very much of the operation conditions and regime, and of the intake cut off decision in compressible fluid mode. Very often, it is not possible or even suitable, to keep a motor running at its optimum efficiency regime.

When a motor is used with compressible fluid in pneumatic, steam, expander or turbo-pump, there is always an inerrant competition between the Power level and the Efficiency. Longer the gas pressure is provided to the chamber cycle, more power it will produce, but less expansion this gas will have within the machine, and consequently less gas relaxation will occur before the exhaust time - An efficiency penalty imposed when high power level is demanded. Optimum would be a perfect reversibility situation, where the chamber is initially and rapidly pressurized (ideally with hot gas) to such a level that expansion ends near the atmospheric pressure and temperature (or condensation condition) at exhaust time. In such a system, increase in Power can be obtain either by increasing the intake pressure, or by increasing the open duration of the intake valve cut-off.

The practice of truncating the intake cycle is common to all positive displacement engines. Old steam locomotive did have some cut off capability once cruise speed attained, as have many of the today vane type motor  (fixed expansion ratio). Of course, his valve must be synchronized somehow with the motor shaft or the chamber cycles, and the in-line gas pressure regulating reservoir must be located before the cutting valves.

One should also remember that heat is produced when compressing a gas, and as much as possible, this heat should not be discarded by cooling the compressor, but rather stored with the compressed air in an insulated pressure tank (if the compressed gas is going to be relaxed within hours of its compression). This heat would then be efficiently used in the adiabatic cooling relaxation process and insure a much better reversibility of the compression-expansion cycle.

For applications using a fix air reservoir (like high pressure cylinder), intake cut off power reduction can generally be compensated by using higher intake pressure, and by doing so, the overall energy available from a cylinder could increase substantially.


Sophisticated Cut Off Valve

Sophisticated optimization can be achieved through the use of time variable cut-off valves at intake ports. Modern electro-mechanical valves can be computer controlled for optimum opening duration in accordance with the power level and other motor running parameters. The main advantage of electronic control is that it allows infinite possibility of dynamic timing and opening duration. The best would be 2 valves, one for each intake port, but linking the 2 intake ports together would require only one valve.

At this time, such an accessory system is not currently offered, and buyers are free to make their own if needed.


Simple Mechanical Cut Off Valve

Simple intake cut off valve can be made by using two perpendicular radial holes in the central shaft as part of the rotary valve. Unless a timing device is incorporated, opening is of a fixed duration, which limits the optimization. A brass or nylon disk with similar perpendicular radial holes being fitted over the shaft, with the 2 exits going respectively to their respective Quasiturbine entry port. Diameter of the holes in relation to the shaft diameter determine the fraction of the time the flow is on. Typically a fraction around 50 to 65 % is a fair test debut. Synchronization timing is done by moving slowly the disk angle position before bolting it in place.


Example of a simple rotary synchronization intake cut off valve,
using 3/16 of an inch perpendicular cross holes on a 3/4 inch diam. shaft.
Valve open 65 to 75% of the time (not variable).

Alternatively, a cam on the central shaft could drive mechanical valves located at both intake ports, with the further advantage of reducing the residual volume between the valve and the chamber (which reduces the geometric compression ratio). Mechanical valve driving train do not offers much flexibility, but it is very worthwhile considering its relative simplicity.

A variation of this simple mechanical cut off valve could be efficiently located at each intake port, providing a proper driver gear and strap to rotate them in phase (fixed opening duration), or better, to be driven by 2 digital electrical motor controlled by computer (variable opening duration), and synchronized from the signal of a microphone listening at the exhaust noise...


Cut off valve located at each intake port.
On the left, the shopper shaft has been removed and is shown for detail.

This accessory system is not currently offered.

Exhaust Pressure Pulse

An other simple mechanical way would be to make use the exhaust pressure pulse to mechanically activate the opening of an intake valve.


THEORY
« Mechanical Power Output » versus
« Intake Cut-off Conversion Efficiently »

(About « multi-stage expander » – See comment below).

« Cut-off expansion ratio » is about the residual exhaust pressure, which could be made zero while the average pressure value could be substantial. If for some reason (like fine rpm control...), the engine power need to be reduced, it could be « done inefficiently » by reducing the motor intake pressure by a flow restriction valve (or dissipative pin hole regulator); or it can be « done efficiently » by cutting-off the high pressure gas sometime after intake opening, and letting the gas to expand somewhat within the motor itself, for a lower average effective pressure, torque and power output.

Typical volumetric expansion cycle.

« Single Stage Expander SSE Model » (A piston in an infinitely long cylinder) is a very convenient theoretical reference to compare expanders between themselves. To pressured load a piston in an infinite long cylinder, not only new gas pressure must be supplied (intrinsic energy, later lost at the exhaust), but once this working pressure is reached, this pressured gas column (then considered as incompressible in transmitting forces) needs to be pushed forward (pressure flow energy) into the cylinder in order for the piston to move and do work. « Single Stage Expander SSE Model » piston does not recover all the incoming pressure energy (without cut-off), because residual gas column built up in front of the piston, and is ultimately wasted in the device exhaust. Intake cut-off allows a pause for the gas to expand and give its intrinsic energy to the piston. « SSE mechanical conversion efficiency » is good at very low pressure, but fall under 42 % over 7 bar (100 psi).

     QUASITURBINE PNEUMATIC / GAS
« Mechanical Power Output » versus « Pressure Conversion Efficiently(*) »
(From Single Stage Expander SSE Model under Iso-thermal approximation)
(Calculated values - Real world can be slightly different...)

  Pressure            Cut-off (chamber %)     Exhaust     Power       Conversion
  (And relative       (And Average               residual     Output       Efficiency(*)
  output power)      feed line flow)              Pressure

  1 bar                  No Cut-Off                  1 bar          100 %        71 %
  1 bar                  50 %                          0 bar           80 %       100 %

  3 bar                  No Cut-Off                   3 bar         100 %        54 %
  3 bar                  25 %                           0 bar          47 %       100 %

  7 bar                  No Cut-Off                   7 bar         100 %        42 %
  7 bar                  12 %                           0 bar          30 %       100 %

(*) « Pressure Conversion Efficiency » refer to the « Single Stage Expander (SSE) » model (A piston in an infinitely long cylinder), and should not be confused with the no cut-off « Comparative Efficiency to (SSE) Model ». However, expander with known « Comparative Efficiency to (SSE) Model» could use this table to estimate their absolute efficiency.

How to read this table:
* From the efficiency point of view: 3 bar (44 psi) operation without cut-off gives 54 % conversion efficiency at 100 % power output, while the efficiency could go up to 100 % if accepting to lower the power output to 47 % (with cut off at 25 % of the chamber volume). Then, adding 3 additional similar expanders to handle the 75 % un-used feed line flow could allow to reach a total power of 200 %.
* From the power output point of view: To keep the output power at 100 %, in order to improve the efficiency toward 100 %, one need to raise substantially the feed line pressure (Then flowing only while intake cut-off valve is open). Such a raise in pressure may displace the motor efficiency on the in-efficient pressure production side, with no global gain?

With intake cut-off, power and efficiency are competing one another, such as no one can get both optimized simultaneously. Higher is the pressure, more potential there is for additional energy recovery by using an intake cut-off. The specific power density of the expander decreases, as the average acting internal pressure decreases under cut-off. Cutting-off more than necessary is simply introducing a dead time in the power output and the intake flow. Notice that « optimum intake cut-off » is a function of the pressure, and the Quasiturbine allows full control on the % of internal expansion,
in contrast with vane motors, that have fixed % cut-off incorporated into an expander design (not variable). Nevertheless, the fixed expansion ration is a minor defect of the vane motor, compared to their large chamber loading losses.

Multi-stages expander
« Multi-stage expander » and « Intake cut-off » are two very different techniques with different impact on power and specific power. Intake cut-off permit higher efficiency with a single expander, but reduced the specific output power (consequence of the intermittent intake flow). Another way to increase power efficiency while keeping the full intake flow (not only for high pressure drop), is to cascade several expanders on a common power shaft, to force near equal stage torque (and power) contribution
(individual expander displacement size going up as pressure goes down). Then, no intake cut-off is used, and the residual exhaust pressure of the last stage dictate the overall efficiency. This « Multi-stage expander » techniques is very feasible with Quasiturbines and optimizes both the efficiency and the power output, but it requires several expanders and is technically more complex and costly.


More Technical

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