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Module 3: Influence of Engine Design and Operating Parameters on Emissions
Lecture 15:Effect of CI Engine Design and Operating Variables on Emissions
Effect of CI Engine Design and Operating Variables on Emissions
The Lecture Contains:
Engine Design Variables
Operating Variables
Compression Ratio
Direct Injection (DI) versus Indirect Injection (IDI) Engines
Combustion Chamber Design
Fuel Injection Timing and Injection Pressure
Engine load and Speed
Exhaust Gas Recirculation
Fuel Quality
Module 3: Influence of Engine Design and Operating Parameters on Emissions
Lecture 15:Effect of CI Engine Design and Operating Variables on Emissions
CI Engine Design and Operating Variables and Emissions
Important diesel engine variables that influence emissions are:
Engine Design Variables
Compression ratio Combustion chamber type Combustion chamber design Injection system: injection pressure and timing, nozzle holes, nozzle sac volume
Operating Variables:
EGR
Engine speed Engine load Fuel quality
Compression Ratio
In the diesel engines the minimum compression ratio that can be used is governed by the ease of engine cold starting ability. For the high speed direct injection engines CR of around 16 to 17.5:1 is used. The turbocharged heavy duty engines employ CR in the range of 13 to 14:1. Cold starting requirements prevents further reduction in the compression ratio.
Use of higher compression ratio results in a shorter ignition delay period. A shorter delay would result in less ‘overmixing' of fuel and air and hence, lower HC emissions. Further, the higher combustion temperatures obtained at a higher compression ratios tend to increase oxidation of the unburned HC.
At a low compression ratio, a longer delay increases the fraction of fuel burned during the premixed phase resulting in higher peak pressures and temperatures which cause an increase in NO x formation. On the other hand, increase in compression ratio due to higher combustion temperatures would tend to increase formation of NOx. If the ignition delay is too long the combustion may begin in the expansion stroke reducing combustion pressure and temperature. Too long an ignition delay leads to lower NOx emissions along with poor fuel efficiency.
Use of a low compression ratio results in too long a delay during engine warm up under cold conditions, and it causes high emissions of unburned fuel which due to its appearance is called ‘white smoke'.
A high compression ratio leading to high combustion temperatures would increase soot formation while on the other hand it increases soot oxidation. For obtaining low particulate and NOx emissions simultaneously, an optimum compression ratio is to be used.
Module 3:Influence of Engine Design and Operating Parameters on Emissions
Lecture 15:Effect of CI Engine Design and Operating Variables on Emissions
Combustion Chamber Design
a. Dead Volumes
Air in the combustion chamber is contained in several different volumes like piston bowl, top land crevice, piston – cylinder head clearance, valve recess and head gasket clearance. Typical distribution of clearance volume at tdc among different components for a DI diesel engine is shown in Table 3.2. The piston bowl in DI diesel engines contains slightly more than 50% of total clearance volume at tdc. The air contained in top land crevice, head gasket clearance and valve recess is nearly 15 % and is poorly utilized during combustion. Even the air contained in the volume between piston crown and cylinder head at tdc is poorly utilized. Piston-cylinder crevice volumes store morethan proportionate air due to lower temperature in the crevice region compared to the temperature of air in the cylinder. A reduction in crevice volume therefore, increases air utilization. Similarly, a lower clearance between piston and cylinder head increases air utilization and reduces the possibility of fuel entering the crevices. Reduction in ‘poor air utilization' volumes results in lower particulate emission and fuel consumption..
Table 3.
Volume Distribution of Combustion Chamber at
TDC
Piston bowl 55%
Piston – Cylinder head 30%
Clearance Valve recess 6%
Top land crevice 7%
Head gasket 2%
b. Multi-Valves and Air Motion
Use of multiple valves (3 or 4) per cylinder increases flow area and hence, the volumetric efficiency of the engine. Four valves per cylinder are now common in gasoline engines. In the direct injection diesel engines use of four valves enables a centralized location of injector and combustion bowl in the piston. The injector can be placed more centrally and vertically. Typical PM-NO (^) x characteristics for 4- and 2- valve per cylinder passenger car diesel engines are compared on Fig. 3.5.With two valves designs, the injector is always offset and inclined. Injector inclination of 20 and 10 degrees from vertical has been observed to give an increase of about 25 and 5 %, respectively in PM emissions due to poor fuel distribution in the cylinder compared to a vertically located injector. Reduction of up to 4 to 7 % in specific fuel consumption is also obtained at the same NO (^) x emission level in a multi-valve engine with
centrally placed combustion bowl and injector.
Figure 3.
PM – NO x trade-off for two valve and four-valve
passenger car DI diesel engines.
A centrally placed combustion bowl has lower swirl requirements and results in more equal fuel distribution and availability of equal air to each spray for mixing. In four-valve engines, symmetrical air motion in the piston bowl and equal fuel distribution between different sprays lead to optimum mixture formation and combustion with very low smoke levels.
Use of lower air swirl in 4-valve engines compared to 2-valve engines for the same PM and NOx emission levels, results in reduction of fuel -air ‘overmixing' during premixed phase of combustion. As ‘overmixing' of fuel is an important source of HC emissions, lower HC emissions in 4-valve engines are obtained.
Figure 3.7 Effect of Injection pressure on PM emissions for a
heavy duty diesel engine.
Module 3:Influence of Engine Design and Operating Parameters on Emissions
Lecture 15:Effect of CI Engine Design and Operating Variables on Emissions
Engine load and Speed
For developing higher power more fuel is injected in the diesel engine thereby reducing excess air, which results in higher combustion and exhaust temperatures. A typical dependence of NO on air-fuel ratio i.e., engine load has been shown in Fig. 3.4. Dependence of smoke on overall fuel-air ratio for a direct injection diesel engine is shown on Fig. 3.8.
Figure
Effect of air-fuel ratio and engine load on soot
emissions for a DI diesel engine
With increase in engine load (increase in fuel-air ratio);
NO (^) X and soot emissions increase.
With increase in engine load as the combustion temperatures increase and oxidation environment for CO is more favourable, CO emissions decrease until excess air reduces to about 30 percent. With further increase in load and fuel-air ratio, CO emissions start increasing again and rise sharply as still more fuel is injected to increase engine power output.
At maximum load, NO (^) x , CO and soot are also at their maximum level. HC however, reduce with increase in engine load as higher gas temperature lead to an increase in the oxidation rates. Engine brake thermal efficiency increases with engine load because the ratio of friction to brake power goes down. Interaction among these factors results in lowest value of brake specific fuel consumption (BSFC), and optimum value of brake specific nitrogen oxides (BSNOx ) and particulate emissions at an intermediate load.
The variable speed engines are designed to give lowest fuel consumption at about 2/3rd of maximum
Module 3:Influence of Engine Design and Operating Parameters on Emissions
Lecture 15:Effect of CI Engine Design and Operating Variables on Emissions
Exhaust Gas Recirculation
The role of EGR is to act as inert diluents and heat sink that reduces the oxygen concentration during combustion and lowers the combustion temperatures. The flame temperatures are reduced as a result of EGR. The NO (^) x formation being an exponential function of temperature, even a small reduction in flame temperature has a large effect on NO (^) x formation. Increase in heat capacity of charge caused by EGR has generally been thought to result in reduction of NOx emissions from SI engines. However, in the diesel engines, EGR can affect NO x reduction in three possible ways. These effects are:
Dilution effect: It is the reduction in inlet charge oxygen concentration Thermal effect: It is the increase in inlet charge heat capacity, and Chemical effect: Modification in combustion process as a result of dissociation of CO 2 and water vapour
The dilution effect (reduction in inlet oxygen) is the dominant effect in case of diesel engines. The chemical and thermal effects are relatively small.
Figure 3.
Typical effect of effect of EGR on NOx, HC and fuel
economy for a turbocharged, intercooled passenger car DI
diesel engine
Typical effect of EGR on NO (^) x , HC and CO emissions for a turbocharged passenger car DI diesel engine is shown on Fig 3.9. At around 10% EGR, 50% reduction in NO (^) x is obtained with little change in CO and HC. As the EGR rate is increased beyond 15 %, NO (^) x decreases further, but CO, smoke and
HC are increased. The excess air declines with increase in EGR causing sharp increase in smoke and loss in fuel economy.
The effect of EGR at the same rate (% of intake air) on diesel NOx is lower compared to SI engine as
the exhaust gas in diesel engine contains smaller amounts of tri-atomic gas CO 2 .. Exhaust gas
recirculation has been used on diesel passenger cars since mid-1990s to reduce NO (^) x emissions. This
is being applied now on more and more diesel engines as the emission standards are being tightened.
Engine speed
Engine load
Coolant temperature
Fuel cetane number
Fuel sulphur content
Fuel volatility
Module 3:Influence of Engine Design and Operating Parameters on Emissions
Questions
(3.1) Calculate average molar specific heat of products of complete combustion of C 8 H 18 with
fuel air equivalence ratio, =1.0 (typical of SI engine) and^ = 0.7 (typical of CI engine
atfull load). EGR is used to control NO formation in IC engines. Using this information
explain why EGR is more effective in SI than in the CI engines? Show qualitatively on
the same graph the trends in percent reduction in NO with EGR rate for typical SI and CI
engine operation. Molar specific heat for the gases; N 2 = 33.75, O 2 = 35.59, CO 2 =
55.37, H 2 O = 44.94 kJ/kmol.K.
(3.2) For a SI engine having the same swept volume its design is changed from square (bore=
stroke) to over square ( bore> stroke) configuration. Discus how will it affect the engine
emissions of HC, CO and NOx. An over square engine can be made to operate at a
higher engine speed. Howl an increase in speed would influence emissions? In all the
cases imep is kept nearly the same.
(3.3) Explain the trends in NO formation observed for DI and IDI engines shown on Fig. 3.4.
(3.4) Discuss why advancement of injection timing in a CI engine results in an increase in NOx
and HC emissions but in lower soot emissions.
(3.5) Following Table 3.3 show the trends on the effect of inlet air humidity, boost pressure,
valve flow area, mean piston speed, EGR and oxygen enrichment of air in the cylinder
