Analysis of calculations to forecast energy of CAT’s vehicles using MDI technology - Energy audit carried out by the department of energetics of l’Ecole des Mines of Paris (July 2003)
Analysis of calculations

to forecast energy of CAT’s vehicles using MDI technology

Energy audit carried out by the department of energetics of l’Ecole des Mines of Paris (July 2003)



INTRODUCTION

An audit of the calculations in order to check the autonomy of the CATs vehicles was carried out at the request of MDI by the department of energetics of l’Ecole des Mines of Paris. This study based on calculations using the thermodynamic properties of air at high pressure was carried out in parallel with an analysis of the results of the 34P01 engine on test bench.
  An audit of the calculations in order to check the autonomy of the CATs vehicles was carried out at the request of MDI by the department of energetics of l’Ecole des Mines of Paris. This study based on calculations using the thermodynamic properties of air at high pressure was carried out in parallel with an analysis of the results of the 34P01 engine on test bench.



MAIN RESULTS


  The objective being to determine an order of magnitude of the autonomy of MDI compressed-air vehicles, calculations were carried out by taking present state of prototype engines into account, and various driving conditions.

According to these many parameters, the results of the autonomy are included between 117 and 146 km (73 and 91 miles), downtown. At stabilized speed they vary, according to the parameters, between 115 km (at 50 km/h) and 670 km (at 20 km/h) = 71 miles at 31 mph, 416 miles at 12.4 mph.

  An estimate of the autonomy integrating the electric consumption of the accessories of the vehicle was also carried out. The values found under these conditions are not to take into account because the CATs vehicles use a battery being used for the electric accessories, that it is refilled during the phases of braking and deceleration like during the filling of the tanks on electrical supply network. The autonomy of the vehicle is thus not affected by the use of the headlights, wipers, or others electrical appliances.

  The search for the main parameters influencing the performances of the vehicle highlighted the fact that the outside temperature does not modify significantly the autonomy and the performances of the car. In the same way, this study showed the variation of the engine output, according to the pressure decrease in the reserves (during operation), is negligible.

The search for the main parameters influencing the performances of the vehicle highlighted the fact that the outside temperature does not modify significantly the autonomy and the performances of the car. In the same way, this study showed the variation of the engine output, according to the pressure decrease in the reserves (during operation), is negligible.

CONCLUSION  The general conclusion of the report of l’Ecole des Mines of Paris confirms the well-founded concept of CATs vehicules by specifying that:

"The global concept permits a significant autonomy of the MDI compressed air car"







École des Mines.



  Analysis of test bench results and calculations

to forecast energy performance

of CAT vehicles using MDI technology


FINAL REPORT

All drawings and pictures of the CAT concept presented in the report are by the courtesy of MDI

Summary

The global CAT car concept is based on several concepts:
  • compressed air engine,
  • versatility of the engine, which is running as a compressor for recharging the air tanks,
  • light urban vehicles,
  • car manufacturing at small scale.


  The only purpose of this report is to evaluate the possible autonomy of the CAT vehicle based on calculations and available test data provided by MDI.

  The design of the actual air engine, named 34p01-4, is a 3-stage air expansion engine having 2 heat exchangers to re-heat the expanded air after the two first expansions.

CAT car autonomy forecast

  • Based on calculations using accurate thermodynamics properties of air at high pressure, Part 1 of the report evaluates the impact on the CAT car autonomy of:
  • isentropic and mechanical efficiencies of the air engine,
  • leakage ratio of the engine,
  • the air temperature,
  • the heat exchanger effectiveness.


  Those parameters are grouped in 4 sets permitting to define 4 levels of efficiencies of the compressed air engine. Evaluations have been realized in steady state conditions for 2 speeds of the CAT car: 20 (12.4mp/hr) and 50 km/hr (31mp/hr).

Gross autonomy

Table 1: Gross autonomy of the car for the four sets of efficiency at 20km/hr (12.4mp/hr)
Distance Low efficiency Average efficiency system Baseline system system High efficiency system
(km) 222 406 509 590
Miles 138 252 316 367


  The autonomies, calculated depending on the different efficiencies, are the gross autonomies because the impact of the electrical consumption of accessories is not taken into account.

Autonomy with electrical consumption of accessories

  Electrical accessories comprise mandatory safety accessories: wipers, lights, warning, and also starting of the air compressed engine. 300W is the maximum electrical power considered to be needed for running the car at night under raining conditions.

  Electrical accessories comprise mandatory safety accessories: wipers, lights, warning, and also starting of the air compressed engine. 300W is the maximum electrical power considered to be needed for running the car at night under raining conditions.
Table 2: CAT car calculated autonomies depending on the speed and the electrical consumption
Tank volume (liters) Baseline system (km/miles) Baseline system (km/miles) Baseline + 300 W (electric) Baseline + 300 W (electric)
  at 20km/hr
(12.4mp/hr)
at 50km/hr
(31mp/hr)
at 20km/hr
(12.4mp/hr)
at 50km/hr
(31mp/hr)
3 x 114 509 / 316 115 / 71 120 / 75 93 / 58
3 x 150 670 / 416 143 / 89 146 / 91 117 / 73


  Table 2 shows the significant impact of electrical consumption of the car, leading to an autonomy in between 117 to 146km (73 to 91miles) with 300W of electrical consumption, and for the 2 levels of the car speed. This autonomy is possible for the baseline case where:

  • the efficiencies of the engine are of 85%,
  • the leakage ratio is of 2%, and
  • the heat exchanger effectiveness is of 50%.


This set of efficiencies corresponds to a good design and realization of the engine.

  Special attention shall be paid to cabin heating in winter conditions. The choice of an electrical heater could compromise the car autonomy down to 35 to 45km (22 to 28miles) if the needed electrical power is of 2kW. A fuel burner for heating seems to be an appropriate solution to avoid such a high electrical consumption.

On the contrary, for the cabin cooling, it shall be underlined that the CAT car concept permits to use free cooling due to the air low temperature (-40°C) at the air exhaust.

Actual tested prototype

  Part 2 of the report analyzes the effective performances reached by the actual prototype, named 34p01-1. Due to delay in the realization, the actual tested prototype is not the complete 3-stage air engine but only the last stage. Nevertheless the tests permit to evaluate the efficiencies and leakage of the actual prototype.

Table 3: Actual performances of the 34p01-1 prototype.
  Parameters
Isentropic efficiency (%) 0.70 to 0.75
Mechanical efficiency (%) 0.70 to 0.75
Leakage on the main air loop (%) 10
Leakage in the cylinder chamber (%) 4 / (6 for 1piston)
Pressure losses (MPa) 1 – 1.4


  The actual efficiencies that have been measured are in the low range. Those levels of efficiencies require significant improvements in order to reach the forecasted efficiencies of the baseline system presented in the previous paragraphs.

  In conclusion, the global concept of CAT cars using compressed air permits to drive small urban vehicles. The design of the 3-stage 34p01-4 engine of MDI permits to forecast a possible autonomy corresponding to urban usage (between 117 to 146km (73 to 91miles)) taking into account typical speeds from 20 to 50km/hr (12.4mp/hr to 31mp/hr). At high speed, the autonomy will be lower.

  To move the project from the design stage to the manufacturing process, a lot of development work is needed in order to reach the level of efficiencies required for the forecasted autonomy.



Introduction

  The objective of this report is to evaluate the energy performances of the compressed air engine developed by MDI.

  The work program covers two main aspects:
  • calculations and analysis of the expansion process,
  • analysis of test bench results.
1. Calculations and analysis of the expansion process

  Based on the actual design of the engine and the working conditions (number of expansion chambers, number of heat exchangers, air pressure inside the tanks, power to be developed by the engine to move the car at various speeds), the expansion process simulation will permit to evaluate the autonomy of the MDI cars for various driving conditions.

  The simulations take into account various parameters such as engine and heat exchangers efficiencies, the volume of the tanks, and the air leaks in order to analyze their relative influence on the car performances.

2. Analysis of test bench results

  MDI has performed various tests on their actual prototype and sent us the available results (power, rotation speed, pressures, temperatures and duration time for different running conditions). The analysis of these results permits to evaluate the energy performances of the actual prototype.

3. Visit of MDI facilities

  Mrs J. and D. of ARMINES have visited MDI facilities in Carros near Nice, on June 2 and 3. Messrs. Guy and Cyril Negre, and their team have presented the different concepts of CAT cars. The visit has permitted to have a demonstration of the actual engine prototype on a test bench. Different workshops of the facilities have also been visited to present the realization of parts of the vehicle.

  Test data on the actual prototype have been provided by MDI and are used in Part 2 of this document.



  Note: Values for conversion
1mile = 1.609km 1kW = 1.359hp (ch) 1bar = 100kPa = 0.1MPa


  Part 1 Calculations and analysis of the expansion process

1.1 Engine presentation

  MDI has developed various compressed air models of engines that can power little city cars.

  The late model is named 34p01-4 and is presented Figures 1.1 and 1.2.

Details of MDI Engine.


2 Expansion and reheat process

  The actual design of the 34p01-4 engine is based on a three-stage expansion process coupled with two heat exchangers. The higher the number of stages in a compression / expansion processes, the higher the efficiency.

Two heat exchangers are used to reheat the air at the outlet of the first and the second expansion stages.

MDI Engine Operations.


  The air reheat between each expansion stage is essential to realize an evolution as close as possible to an isothermal expansion and as far as possible from an adiabatic one. In fact an isothermal (expansion or compression) presents a better efficiency than an adiabatic one. In he case of the 34p01-4 engine the two heat exchangers aim at achieving the compression process closer to the isothermal expansion.

1.5 General conclusions

  MDI has developed the global design of the 34p01-4 engine, which is not running yet. A significant work for optimization and control is still necessary to lead to an available prototype. Moreover, the electrical consumption of accessories needs a specific design.



Nevertheless, the global concept permits a significant autonomy of the MDI compressed air car