Introduction
1.3 PERFORMANCE IMPROVEMENT METHODS
The spark ignition engine with different octane fuels needs to be utilized to their optimum performance. In order to achieve this it becomes necessary to rethink about the structural alterations during running of the vehicle as per load and acceleration on the engine. The attempt has been made in this thesis towards some performance improvement techniques as detailed herein.
Table 1.5 A comparison of some physical properties of engine fuels
Properties LPG Biogas Petrol
Molecular formula C3H8 CH4+CO2 C8H18
Density at 150C 507 1.1 730
Lower heating value, MJ/kg 46.10 25 44.0
Heat of vaporization, MJ/kg 0.426 0.5 0.33
Stoichiometric A/F ratio 15.7 9.5:1 14.7
Research octane number 112 110 91
1.3.1 Variable compression ratio
The technique of variable compression ratio is quite old since 1920 where Harry Ricardo developed an engine with varying compression ratio. He paid attention towards the variation of clearance volume inside combustion chamber. In doing the work, Ricardo forms the Octane rating system for the fuel which still being used. But the VCR engine development was not successful due to mechanical complex manufacturing and difficulty of controlling all of the parameters. The advantages of the increase in compression ratio, increases the power output and efficiency. This is due to higher pressure intern higher temperature of compressed fuel- air causing complete combustion of fuel air mixture in cylinder. The compression ratio may vary for different octane fuels, different load conditions and different speed as represented in Table 1.6 reported by different researchers based on their exhaustive experimentations. So it is a challenge to find the optimum compression ratio which gives maximum power, efficiency, lower bsfc and lower emission.
Table 1.6 compression ratio with varying the octane rating of gasoline.
Octane Rating 72 81 87 92 96 100 104 108 Compression ratio 5:1 6:1 7:1 8:1 9:1 10:1 11:1 12:1
There is a maximum compression ratio limit beyond which the knock may occur (Crookes, 2006). The renewable fuels and non-renewable fuels discussed above such as petrol of different brands, the xtra premium petrol, power petrol and such various grades, gaseous fuel such as LPG, raw biogas consist of various different compositions of hydrocarbon , different additives. This fact suggests that there is different engine combustion environment for maximum performance with each. The variable compression ratio is one of the technique whereby the CR can be optimized for each different fuel in the same SI engine (Cerri et al., 2013). The advantage of this is the high specific power out put accompanied by good reliability and longer engine life.
The optimum CR for different liquid and gaseous fuels reported by authors is as tabulated in Table1.7. It is very clear from this discussion is that, the compression ratio variation is important in spark ignition engine with variation in fuel being supplied to the engine. For the spark ignition engine, the compression ratio is made fixed at a particular optimum location as per the fuel that is supplied to the engine. From this, one must ensure that the fuel performance is optimum. However when engine operates, there are certain other parameters which needs to be accommodated for optimizing the compression ratio. Those parameters are load on the engine and ultimately the speed of engine. From literature, it was observed that with the lower load higher compression ratio should be maintained and vice-a-versa during higher load condition. The high peak pressure problems occur at high load, so need to reduce the CR whereas during part load or cold starting condition high CR is recommended.
Table 1.7 The optimum CR for variety of fuels and engines reported by researchers.
Sr
No Type of Engine Fuel used Optimum
CR Reference
01 12.78 kW Ricardo E6 petrol engine@3000 rpm
Simulated biogas 13:1 Crookes,2006 02 3.5 kW diesel converted to petrol
engine@1500 rpm
Raw biogas 13:1 Porpatham,2007 03 3.5 kW diesel converted to petrol
engine@1500 rpm
Simulate biogas 12:1 Chandra et al.,2012
Methane 13:1
Raw biogas 13:1 04 58.88 kW, 4 cylinnder,4-stroke,
1.4 ltr SI engine@1200-2300 rpm
91 RON
Gasoline
8:1 Sayin, 2010
93 RON
gasoline
8:1
95.5 RON
gasoline
8:1 05 12.78 kW Ricardo E6 petrol
engine@3000 rpm
Power gas(CO+H2)
8:1 Mustafi et al.,2006 06 191 kW ,6cylinder SI
engine@2300 rpm
Biogas with hydrogen
10.5:1 Park et al.,2011 07 107 kW, 4-stroke, 5 cylinder
FIAT engine@6100 rpm
LPG 10 Masi, 2012
The method of continuous variation of CR adjusting the clearance volume and swept volume of combustion chamber will definitely achieve the maximum performance from the engine.
Many researchers worked in this regards but still the vehicle with VCR implemented technology is a dream.
1.3.2 Optimum Ignition timing or spark timing
The ignition timing of the fuel-air mixture is important for the best performance of the engine. The ignition timing is assumed to be the spark timing of the spark plug. This spark timing may be advanced or delayed of crank angle before TDC. During combustion for much advanced spark timing the cylinder pressure increases substantially before compression stroke complete and hence useful compression work reduces. Much delayed spark timing reduces the peak pressure for expansion stroke so expansion work reduces. Due to this, the optimum spark timing is very important. The spark timing optimization should be focused for some important parameters such as type of fuel, its octane rating, its heating value.
Depending upon the phase of fuel whether liquid or gaseous the ignition timing may be optimized. Even the octane rating matters a lot for optimizing the spark timing. Higher octane fuels needs to settle the ignition timing to avoid knock causing vibrations and shocks. In this regards, the engine control management advances or retarded the ignition timing based on the signal provided by knock sensors fixed at different places inside combustion chamber. The
quality of fuel also becomes an important parameter for optimum spark timing to have homogeneous combustion of charge with maximum efficiency.
Table 1.8 Indian emission standards for four wheelers (Emission Standards, 2017)
Standard Reference Date Region
India 2000 Euro 1 2000 Nation wide
Bharat stage II Euro 2
2001 NCR*, Mumbai, Kolkata, Chennai
April, 2003 NCR*, 11 Cities$
April, 2005 Nation wide
Bharat stage III Euro 3
April, 2005 NCR*, 13 Cities$
April, 2010 Nation wide
Bharat stage IV Euro 4
April, 2010 NCR*, 13 Cities$
April,2017 Nation wide
Bharat stage V Euro5 To be skipped
Bharat stage VI Euro6 April,2020(proposed) Entire country
* National Capital Region (Delhi)
$ Mumbai, Kolkata, Chennai, Bangaluru, Hyderabad, Ahmedabad, Pune, Surat, Kanpur, Lucknow, Solapur, Jamshedpur and Agra
1.3.3 Optimum spark plug location
The spark ignition engine which are generally designed and developed under impression of the fuel being used is petrol. The properties of petrol fuel are known and accordingly the spark plug is located inside combustion chamber. This location of spark plug is optimum from the point of view of combustion chamber design, petrol fuel in regards to the flame kernel development and faster flame travel towards opposite corner of combustion chamber.
In the conventional engine, if the fuel other than petrol is used for combustion, then the issue of combustion duration arises as well as corresponding mass fraction burned as a function of crank angle displaced. Flame speed of the fuel air mixture inside combustion chamber depends largly on compression ratio, ignition timing as well as ignition location and that will be different for different fuels.
Many researchers worked in this area of optimizing the spark plug location for variety of fuels. The engine performance will be maximum if all three parameters CR, I.T. and ignition location are adjusted at the optimum magnitude for the particular fuel (Yamin and Badran, 2000).
The research of varying the spark plug location from centroidal axis towards wall of cylinder were attempted as well as some researchers are working on laser based spark ignition which can be concentrated at position inside combustion chamber as per fuel in use, load condition and CR (Pal and Agarwal, 2016).
Table 1.9 Emission standards for city cars, 3-wheel and 2-wheel petrol vehicles, g/kWh (Emission Standards, 2012)
Year of assessment
1991 1996 1998 2000 2005$ 2010$ 2010$$
Reference - - - EURO1 EURO2 EURO3 EURO4 Emission(Gros
s vehicle weight≤3500
kg),g/km
CO 14.3-
27.1
8.68- 12.4
4.34- 6.20
2.72- 6.90
2.2-5.0 2.3 4.17 5.22
1.0 1.81 2.27
HC 2.0-2.9 - - - - 0.20
0.25 0.29
0.1 0.13 0.16
HC+NOx - 3.00-
4.36
1.50- 2.18
0.97- 1.70
0.5-0.7 - -
NOx - - - - - 0.15
0.18 0.21
0.08 0.10 0.11 Emission for
3-wheel petrol vehicles, g/km
CO 12-30 6.75 - 4.0 2.25 1.25 -
HC 8-12 - - - - - -
HC+ NOx 12-30 5.50 2.00 2.00 1.25 -
Emission for 2-wheel petrol vehicles, g/km
CO 12-30 5.5 - 2.0 1.5 1.0 -
HC 8-12 - - - - - -
HC+ NOx - 3.60 - 2.22 1.5 1.0 -
+ for catalytic converter fitted vehicles
$ Mumbai, Kolkata, Chennai, Bangaluru, Hyderabad, Ahmedabad, Pune, Surat, Kanpur, Lucknow,Solapur, Jamshedpur and Agra