DISCUSSION OF RESULTS
5.1 Running the Engine on Petrol
The performance tests of the engine-generator set runnIng on petrol (the fuel for which it was designed) at different electrical loading conditions were initially intended for generating the base data, against which the performance with natural gas could be compared. During these tests some interesting infonnation were revealed to us regarding the operation of these small generator engine, often which are not highlighted by the manufacturer.
[n general the small engine-generator set was found to be operating at low overall efficiencies, only about 11-12 % near rated capacity. This also was found to be decreasing sharply under part load conditions. The brake specific fuel consumption was very high at part load and still about 690 glkW-h near the rated capacity. This relatively poor burning and high fuel consumption rate was exhibited in the A/F ratio curve and the emission characteristics. The A/F ratio was found to be nearly constant throughout the power range, indicating the elTectiveness of the fuel governing system for a constant speed engine. The relatively low value of A/F ratio near 10 (stoichiometric value is about 14.8 for leaded petrol), indicates a rich mixture being used which would result in a high fuel consumption rate. The calculated AlF characteristics was closely supported by the exhaust gas analyzer equivalent ratio readings.
The 4-gas emISSIon analyzer results showed high proportions of CO generation (around 10%) and only about 7-9% CO2 generation, indicated the incomplete combustion of the fuel. The unburnt
He
contents were found to be above 500 ppm, throughout the range. The variation characteristics of all the emission data were found to have a stable trend with slight improvement in emission results beyond 50%of the rated capacity. The engine tuning was readjusted, but the results found were very much similar. All these suggested that such small generator engines are not very efficient mechanisms of converting enerb'Yof fuel to electricity. For verifying the rcal
picture, catalogue specification of two other generator brands (HONDA EM-IOOOF and ALPHA AL-G 2500) of similar power ratings were studied, which revealed brake specific fuel consumption rates of similar scale (448 and 537 g/kW-h respectively) at rated capacity. Although for all the three brands investigated, generators with higher power rating were found to have much improved fuel consumption rate.
Another interesting finding was that although these generators imported to Bangladesh are rated as capable of producing 220V electricity, practically under loading condition the voltage was never found to be 21 OV and dropped down nearly by 10%, for loads beyond 80% of the rated load capacity. This drop of voltage was found to increase as soon as the generator's rated current capacity of 6.8 amps was exceeded. Compared to this the fluctuation of the engine speed did not exceed :1:2%.
The variations in lubricating oil and engine body temperatures were found to be within 25°C, while the exhaust temperature increased by about 75°C through the power range.
5.2 Running the Engine with Closely Regulated Natural Gas Flow
In this phase the generator was run on natllTal gas with only modification being in the intake manifold. The gas !low was closely regulated manually to suit the different electric loads applied. The performance of the engine running with gas showed improvement in several aspects compared to operation on petrol. The AIF ratio was found to be between 16-21 (stoichiometric ratio for NG is about 17.2), supported by the analyzer data. The air flow rate was found to he about 55-65% of that of petrol.
This must have been caused by the relatively l,'Teatervolume occupied by the gas, fed into a fixed speed engine. The overall efficiency was found to be improved reaching aboot 16% at rated capacity. This apparent improvement in overall efficiency has more to do with the improper/incomplete combustion of petrol, rather than real enhancement of efficiency using natural gas. The higher air flow rate for petrol indicates that improper mixing rather than lack of air, most probably is the main cause of its higher fuel consumption rate. The benefit of improved brake specific
of the two fuels in Bangladesh. Petrol being an imported fuel is about 6-10 times (Considering gas price applicable for commercial units or small industries) more costly compared to price of indigenous natural gas supplied to small commercial units. Higher environmental pollution rates of petrol compared to natural gas also makes it more prone to be taxed in near future from environmental point of view.
The most significant improvement regarding emission characteristics is the drastic reduction of CO emission to a minimum using natural gas. The generation of CO2 gas had increased to about 12%, both indicating improved combustion of the fuel.
The unbumt HC content was also found to be lower compared to petrol.
The reduction of the airflow rate resulted in limiting the maximum power to be produced inspite of the better combustion. Improper spark advance relating to slower flame propagation speed for natural gas also may have contributed to this. The engine was found to be capable of producing nearly 85% of the rated capacity with petrol fuel. Beyond this limit the engine performance was found to be deteriorating in terms of bsfc & overall efficiency, voltage generated, engine speed as well as noise and vibration.
No significant change in the variation of lubricating oil temperature and engine body temperature (only about ]00C) was noted with natural gas, although the exhaust temperature was 30-40°C higher. This was promising, indicating less possibility of variation in expected longivity of engine parts, when using natural gas. The noise and vibration levels were found to be of very similar order for both fuels, in the load range where the engine was operating smoothly.
Although the maximum load capacity was found to be some what limited by using natural gas and the need of controlling gas flow added complexity, the improvement in overall efficiency and significantly better emission characteristics definitely justifies the use of natural gas. In addition the economic as well as strategic aspects of using an indigenous fuel makes natural gas a very potential alternative fuel for Bangladesh.
5.3 Running the Engine with Single-Step Natural Gas ~Flow
The performance of the engine-generator set usmg natural gas could be further improved by modifications and proper adjustment of several engine components like _ the spark timing mechanism, throttle control systcm and engine cooling system. But these would involve additional cost as well as loosing the present advantage of instant inter-changability of fuels. Compared to the reduction of rated capacity (about 15%)- the lower fuel cost, improvement of combustion and better emission characteristics may encouragc thc end uscrs not to go for full conversion to natural gas. The only sib'Tliticant inconvenience for running the engine with natural gas may rise from the controlling aspect of the gas (specially for users having variable electrical10ads). The single-step flow regulator is an approach in that direction, relieving the operator from controlling gas flow at different loads.
The suitably tailored single-step flow regulator was found to be operating well from 50% up to 85% of the rated capacity. As the fuel flow rate was kept almost same through out the working range, it could be perfectly matched only for a limited load range, beyond which the engine may not be running that efficiently. At part load the overall eftlciency was found to drop by 3-4 %, using this system. Using this system some of thc improvements in emission characteristics were sacrificed, specially CO emission raising up to nearly 4% at 50% capacity, although still significantly less compared to the petrol counterpart.