mercoledì 8 agosto 2007

RENDIMENTO MOTORE ARIA COMPRESSA






Ritorno sulla mia "passione" per il motore ad aria compressa dell'Ing. Di Pietro, sono riuscito ad ottenere un documento riguardo il rendimento del motore in se, lo riporto di seguito in Inglese.

Non mi azzardo in una traduzione perché non sono ben sicuro di tradurlo correttamente... se qualcuno più preparato di me si volesse cimentare sarò lieto di pubblicarne anche la traduzione.

Executive summary

This report details the technical capabilities of the Di Pietro engine design and explores the possibilities for its further improvement and development with the new design.

The Di Pietro concept is capable of converting theoretical potential work into engine output at 85% efficiency with very high torque and minimal energy losses.

The new engine design and the new operating strategy has the potential to deliver energy efficiency at the rate of 250 J/g of compressed air as compared to state of the art existing air motors, which deliver work at the rate of 42 J/g.

At this rate of power delivery compressed air could compete with lead acid battery which stores energy at the rate of 100 J/g.

A thermodynamic model of the engine was developed and used to study the performance of the existing prototype engine and its derivatives in a variety of potential applications.

Overview of Engine Capabilities

Di Pietro Engine - Description

The Di Pietro Engine consists of an orbiting shaft driver and outer stator. The space between the stator and shaft driver is divided into segments by pivoting dividers which together create air chambers. The compressed air pushes the shaft driver towards low pressure chambers, which causes the shaft to rotate in that direction. The unique feature of this engine is its very high torque at standstill. It has further advantages of simplicity of construction, very low friction and low spinning mass. All this, combined with its high power to weight ratio makes it an ideal candidate for direct drive applications.

Characterising the Di Pietro motor

A thermodynamic spreadsheet model of the Di Pietro engine operation was developed and used to compare theoretical (modelled) results with the claimed performance. Engine Performance Test Results from Monash University were used to construct engine operating diagrams. These are summarised in Figures 1-3.

Power

The results presented in Figure 1 show that the Di Pietro engine, depending on inlet pressure reaches the maximum power output at around 2000 rpm. The power output decreases at higher engine speeds.

Torque

Torque is the most important characteristic of any spinning propulsion system. In most applications, the most critical torque is from standstill to about 60 rpm. As most motors do not have significant torque at low revolutions, most applications, especially those involving low speed.

The torque produced by the Di Pietro engine, shown in Figure 2, decreases linearly with engine speed. The results suggest that the Di Pietro engine, depending on inlet pressure (95 psi or 136 psi) has a maximum torque of about 41 and 57 Nm at standstill respectively. At peak power of around 2000 rpm, the engine torque to power ratio is 5 Nm/kW. At very low speeds the torque to power ratio is in excess of 50 Nm/kW. This compares favourably with internal combustion engines, which typically have a low torque at low speeds and maximum torque at speeds in excess of 3000 rpm. The torque to power ratio of i.c. engines at maximum power is about 2. These results suggest that the Di Pietro engine may be suitable for direct drive applications and thus could eliminate the need for clutches and gear boxes in a very wide range of applications.

Air consumption

The Di Pietro Engine has a nominal swept volume of 6 chambers of 266 cm3 and this was used initially to calculate the theoretical air consumption at respective engine speeds. The results summarised in Figure 3 show that the actual air consumption exceeds the theoretical by between 69% at 750 rpm and 13% at 2100 rpm. The excess air consumption could be explained by leakages, but it also includes all the air contained in the feeding and the distribution system which is exhausted on emptying of each of the 6 chambers. Not all the excess air consumed represents inefficiency. This will be discussed later. The decrease in excess air consumption with increasing engine speed suggests that the actual chamber pressure at high speeds may be significantly lower than inlet pressure. This gives rise to lower air consumption, lower effective pressure and hence a loss of power is observed. Therefore, the observed loss of engine performance at high speeds is probably related to air ingress and egress design, rather than an inherent limitation of the engine.

Overview of Engine Capabilities

Brief description of the model

The operation of the Di Pietro engine can be modelled as three critical stages: air injection (isostatic), expansion (adiabatic) and air expulsion. Each of these stages may involve mechanical work and frictional losses. The model calculates from first principles: pressure loss due to fluid friction, thermal changes due to expansion of gas, pressure changes and work done on the shaft driver. The model also calculates total thermodynamic work required to produce the compressed gas, i.e. work of compression and any additional heating which may be involved. This model was tuned by enlarging the effective chamber volume until the model predictions matched the test results from Monash University.

Note on modelling

An initial attempt to model the operation of the Di Pietro Engine as designed, produced incongruent results. Modelling predicted air consumption which was much lower than observed and power output which was greater than theoretically possible. This indicated that the working air chamber had to be enlarged to include all air which may be contributing to production of work, including the “leaking” air. Therefore the chamber size was arbitrarily increased until predicted air consumption at any engine speed matched the observed.

Model Validation

The model was validated by comparing the predicted and actual air consumption, predicted and measured power output and predicted and measured torque. The results presented in Figure 4 show that the predicted and observed air consumption at different speeds match closely when the effective chamber volume in the Di Pietro engine is increased to 492 cc.

The comparison of predicted and actual power output from the Di Pietro engine operating at 95 and 136 psi inlet pressure is shown in Figure 5. The predicted power output results for the 492 cc “mathematical” chamber volume matches the measured values from Monash University testing.

The comparison of predicted and actual torque output from the Di Pietro engine operating at 95 and 136 psi inlet pressure is shown in Figure 6. The predicted torque results match closely measured results.

As air consumption, power output and engine torque are calculated from first principles and congruent with the data, this Thermodynamic model of the Di Pietro engine operation, developed by D. Technologies is considered valid and was used for engine evaluation. Model can be used to predict the performance of any potential engine or compressor design.

Overview of Engine Capabilities

Design Efficiency of the Di Pietro motor

The theoretical possibility of conversion of the energy contained in a compressed gas to mechanical work in an adiabatic engine is about 75 %. In practice however, the conversion efficiency is often lower than 30%. The modelling results presented in Figure 7, show that although the thermodynamic efficiency of the Di Pietro engine is relatively low, around 35%, its conversion of theoretical potential energy into work is remarkably high at about 85 %. The results also show that the engine as operated in these tests, which involved full chamber injection, with no provision for expansion, was the main reason for low thermodynamic efficiency. The theoretical “design” efficiency under these conditions was low at around 45%. A significant reduction of conversion efficiency with injected air flow rates above 1000 l/min, suggest that the Di Pietro motor as constructed, is most efficient when operated at speeds below 1000 rpm. Absolute majority of possible air motor applications require speeds below 1000 rpm.

Potential for improved Power Conversion – existing engine design

The results presented in Figure 8 show that the Di Pietro Engine as tested is about 40% more efficient than its commercial competitor as measured by air consumption at constant power. It further shows that by way of reducing the amount of air injected into the chamber and allowing it to expand before exhausting, the motor would become much more efficient without loosing significant power. Any power loss from reduced air usage could be recovered by using higher inlet pressures. The use of higher inlet pressures would further enhance motor performance. The model predictions suggest that the existing motor could be made at least 4 times more efficient for the same power output, compared to its commercial competitor
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