Proceedings of the 6th International Mechanical Engineering Conference & 14th Annual Paper Meet (6IMEC&14APM) 28-29 September 2012, Dhaka, Bangladesh

IMEC&APM-AM-04

© IMEC & APM 2012 1 AM-04

1. INTRODUCTION A compressed-air engine is a pneumatic actuator that

creates useful work by expanding compressed air and

converting the potential energy into motion. A pneumatic

actuator is a device that converts energy into motion. The

motion can be rotary or linear, depending on the type of

actuator. Compressed Air Engine (CAE) are fueled by

compressed air, which is stored in a tank at a high

pressure of about 30 MPa. The difference between the

compressed air engine and IC engine is that instead of

mixing fuel with air and burning it to drive pistons with

hot expanding gases, CAE’s use the expansion of

previously compressed air to drive their pistons.

The greatest advantages of compressed air vehicle are no

burning process and no waste gas discharge to the

surrounding environment. It can be said as a green

environmental protection vehicle with near zero

pollution (1) in the metropolitan cities. With the policy of

energy conservation and environment protection, studies

on air powered car become more and more warming-up.

Recently, its researches in domestic and aboard have

achieved notable achievements. Zhejiang University and

Hefei University of Technology of China have

accumulated abundant experience in theory analyses and

experimental test. The MDI Company in France has

realized the design, manufacture, and application of air

powered car (2) (3). The engines of compressed air cars

are piston type, vane type, rotary type and so on, and the

piston engine is widely taken now (4) (5). At present, the

piston engine power system has some disadvantages

such as complex structure, easy wearing, high noise and

low efficiency. Therefore, to develop and optimize

engine power system is the key technique for compressed

air car.

2. PROJECT OVERVIEW In the present study, emphasis was given to

conversion of a two stroke single cylinder SI engine into

a compressed air engine with minimum possible

modification of the existing design. The design is based

on a rather simple working principle.

The compressed air is the source of energy that is stored

in a high pressure cylinder. Basically this cylinder is

re-filled by the compressor. But in the present case, the

compressed air is supplied directly from a compressor at

a pressure of 7 bar. When the piston is at TDC (Top dead

centre) then the inlet cam allows the inlet follower rod to

open the inlet port and let the compressed air enter into

the air chamber. In this situation the exhaust port is

closed by the respective follower controlled by the

exhaust cam. The high pressure air introduced to the

chamber passes through the inlet passage and creates a

downward thrust on the piston and the piston starts

moving downwards. After pushing the piston downwards

the air is then reflected towards the other passage of the

chamber and meanwhile the exhaust cam opens the

exhaust port to leave the air by the use of a follower rod.

The inlet cam has to close the inlet port with the help of

the inlet follower before the piston reaches the BDC

(Bottom dead center).

An extra supply of power is needed to give the piston an

Modification of an Si Engine into a Compressed Air Engine to

Work with Compressed Air or Gas

1Dr. Maglub Al Nur, S.K.M.Asikul Islam

, Debashish Saha and AashiqueAlam Rezwan

1Department of Mechanical Engineering,

Bangladesh University of Engineering & Technology, Dhaka, Bangladesh

ABSTRACT The environmental pollution in the metropolitan cities is increasing rapidly mostly because of the increased

number of fossil fuel powered vehicles. Many alternative options are now being studied throughout the

world. One of the alternative solutions can be a compressed air powered vehicle. The present study focuses

on converting an SI engine into a compressed air engine. A two stroke single cylinder SI engine is converted

to operate using compressed air because of its design simplicity. The cylinder head of the engine is

modified by replacing the existing cam with a set of newly designed flank cam and an inlet-exhaust air

passage. As the cylinder head of the engine is modified, the engine is tested for different timing of the valve

opening and duration of opening. Three sets of valve timing are used to test the converted compressed air

engine. The air pressure used in the present study is 7 bar which is obtained using a compressor. The design

and experimental test result presented here can be used for further research and modification of the

technique.

Keywords: Compressed Air Engine; Valve Timing;

© IMEC &APM 2012 AM-04 2

upward motion after it reaches BDC. Therefore the

reciprocating motion of the piston is influenced by the

application of a flywheel mounted on the extended crank

shaft. The power storage capability of the flywheel is

used to supply the extra energy to the piston movement.

The transmission of power from the crank shaft to the

cam shaft is done directly by a chain sprocket mechanism.

The mechanism of cam shaft rotation is done by

mounting the shaft on two ball bearing holders. The two

follower rods are mounted on separate springs placed

above the air chamber. The restoration of force carried

out by spring allows the follower to lift back to its initial

position when the cam position is at zero displacement.

The use of a one way valve is to ensure a unidirectional

flow of the high pressure air from the compressor to the

air chamber.

3. STRUCTURAL DESIGN In the present study, a CDI H1100S motorcycle

engine has been used for modification into compressed

air engine. There are several structural change required

to be done to run the engine with compressed air.

3.1. Extended Cylindrical Chamber

The cylinder head on top of the engine cylinder has been

replaced with a new cylindrical chamber made of mild

steel with pressure coating. The hole provided for the

spark plug has been bored further to match the outer

diameter of the new cylindrical chamber. The cylinders

have two through holes (diameter = 15 mm) for the

intake and exhaust valves to operate. At 60 mm from the

top of the cylinder, two 10 mm holes were drilled for the

purpose of intake and exhaust ports.

Fig. 1. Cylindrical Chamber

3.2. Inlet-Exhaust Passages

To avoid complexity in the design, providing inlet

passage with bypass air chamber as before, whereas

keeping the exhaust passage open the whole time, which

aroused ambiguity in operation when tested the previous

design, two separate passages has been bored in the

newly designed air chamber. A solid partition has been

provided in between the through passages for the safe

operation of valves.

3.3. Cam Shaft

It is a 10 mm diameter solid shaft. The shaft is supported

on two ball bearings staged in their housing and linked

with the output shaft of the engine via chain and

sprocket.

Fig. 2. Cut-way view of the cylindrical chamber

3.4. Inlet-Exhaust Valves

These are basically two mild steel shafts of varying cross

section combining the action of both: a flat faced cam

follower and a valve. The varying cross section of the flat

face valve sits on a spring leaving the rest of the valve

hanging inside the air passage.

Fig. 3. Inlet Exhaust Valve

The cams are tightly attached to the shaft with the help of

Allen key screw.

3.5. Flywheel

A 150mm diameter and 12mm thick mild steel disc has

been cut in lathe machine and attached to the previously

modified output shaft. These dimensions were obtained

from the engine specification (Engine torque = 60 N-m)

and some assumption made according to engine space

constraints. The weight of the flywheel is 6kg.

4. CAM DESIGN For replacing the original cylinder head, a new set of

two flank cams has been designed for operating the inlet

and exhaust valves of the modified engine. Both the

exhaust and inlet cams are symmetric about the center

line of the cam shaft. The cams are made of mild steels.

These cams are cut out from 90mm diameter 40 mm

thick cylindrical discs on milling machine and lathe

machine on the basis of the full scale drawing provided to

get the specific cam profile obtained from calculation

and theoretical assumptions. These cams provide

determined motion to the follower based on the assumed

cam profile which is done in actual practice.

The inlet cam profile is designed to give the valve a

maximum ascent/descent of 18 mm while it is 15mm for

the exhaust cam. The inlet cam has an ascent angle of 40°

and a descent angle of 320°, while the exhaust cam has

an ascent angle of 150° and descent angle of 210°.

Inlet air

passage

Inlet port

Exhaust air

passage

Exhaust

port

Exhaust

port

© IMEC &APM 2012 AM-04 3

Fig. 4. Inlet Cam

Fig. 5. Exhaust Cam

4.1. Valve Timing

The converted compressed air engine has been tested for

a set of three valve timings. In the first case, the inlet and

exhaust cam are set in symmetric in angle in both side of

TDC and BDC respectively. The inlet cam gives a lift of

18mm to the follower. When the piston is at TDC the

inlet valve is at fully opened condition and as the

compressed air starts entering into the chamber, the valve

has to close the inlet port of 10mm diameter before the

piston reaches BDC. To ensure the complete closure of

the 10mm inlet port, the inlet follower is given a

movement of 18mm. When the crank is at 20 degrees

after TDC, the closure of the inlet port is started by the

valve and it is completely closed when the crank is at 45

degrees after TDC. During this period the inlet port

remains fully closed and no air is allowed to pass through

the inlet port. The inlet port starts to open and allow the

air to pass into the cylinder just 45 degrees before the

TDC and reaches its maximum opening condition 20

degrees before TDC.

The exhaust port is at fully closed condition when the

piston is at TDC and the exhaust valve starts to open the

exhaust port just 75 degrees after TDC. For the next 210

degree rotation of the crank, the exhaust port is kept at

fully opened condition and the air is allowed to leave the

chamber. The exhaust follower is given a displacement

of 15mm to completely close the outlet port of 10mm

diameter. Exhaust valve starts to close the outlet port just

75 degrees before TDC and it is moved to completely

closed condition just when the piston is at TDC.

Fig. 6. (a) Valve Timing Diagram 1; (b) Valve Timing Diagram 2

Fig. 7. Valve Timing Diagram 3

With the modification a second valve timing was tested

with inlet valve starts to open at 25º before TDC and

fully open at TDC. The total opening time remains the

same at 40º of cam rotation. The exhaust valve starts to

open at 60º before BDC and closed after 30º of cam

rotation before the piston reaches the TDC.

With the modification a third valve timing was tested

with inlet valve starts to open at TDC and fully open at

25º after TDC. The total opening time remains the same

40º of cam rotation. The exhaust valve starts to open at

30º before BDC and closed when the piston reaches the

TDC.

5. TESTING RESULTS The engine has been tested with compressed air of 7

bar pressure. In the first design of the valve timing

diagram, the engine starts running with the opening of

the compressed gas line. But after a full cycle the engine

gradually slows down and eventually stops. This may

have occurred due to the fact that in the return stroke, the

inlet valve opens before TDC. This acts against the

piston in the return stroke and eventually slows down the

engine.

In the second valve timing design, the engine starts

running after the compressed air has started to flow into

the engine cylinder. But after several full cycles the

engine again slows down and stopped. In this case, the

inlet valve is started to open before TDC. As the

compressed air started to flow into the engine cylinder,

before the piston reaches TDC in the return stroke, the

compressed air again pushes the piston back in the return

stroke. Thus the engine again slows down and stopped.

In the third valve timing design, the engine starts running

with compressed air flow into the cylinder. This time, the

engine runs for half an hour before the pressure of the

compressed air dropped below a certain limit. The rpm of

the engine has been measured and is found to be 420 rpm.

6. CONCLUSION In the present modification project, a two stroke single

cylinder SI engine has been reassembled with new

cylinder head and cam. The engine has been tested with

three different valve timing diagram. For first two

simplified valve timing, the engine didn’t run for long

time. Thus the design of the valve timing can’t be used

for the modified engine. The valve timing diagram has

© IMEC &APM 2012 AM-04 4

been further modified and with the third valve timing

diagram. This time the engine runs until the gas pressure

dropped. The maximum rpm of the engine is 420.

In future, further modification can be done so that the

engine itself can run the compressor. The engine can also

be attached with other engines in support to the main

engine. The high pressure gas exhausted from the

primary engine can be used as input air for the

compressed air engine. The pressure of the air can also be

increased up to the limit of the engine structure. This

kind of engine can be used for different purposes.

6. REFERENCES

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Time Control of Motors using a Pneumatic H-bridge.

Control Eng Pract. 2001, pp. 449-457.

[2] Huang, K, D, Quang, K, V, Tseng, K, T. Study of

Recycling Exhaust Gas Energy of Hybrid Pneumatic

Power System with CFD. Energy Converstion

Management. 2009, pp. 1271-1278.

[3] Thipse, S, S. Compressed Air Car. Tech Monitor.

2008, pp. 33-37.

[4] Liu, X, Y, Wang, Y. Overview of Developement of

Compressed Air Engine. Machine. 2008, Vol. 35, pp. 1-5.

[5] Shen, Y, T, Hwang, Y, R. Design and

Implementation of Air Powered Motorcycles. Applied

Energy. 2009, Vol. 86, pp. 1105-1110.