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ABSTRACT
Future internal combustion engines for light duty applications will have to cope with a very complex set of customer, legal and business requirements. durability, reliability, drivability, fuel economy, and cost of ownership etc are the digfferent area where the customers would like improvements. DIesel engines will have to cater to future emission standards at affordable cost. airhandling/boosting and control system are the focus areas. fuel economy improvements through improved combustion
systems will be the focus for petrol engines. high power density, low manufacturing cost, recyclability etc should be maintained while reducing the emissions.
THE DIESEL EMISSIONS CHALLENGE
These kind of engiens must cop up with future emission targets at affordable cost. Euro Stage 4 standard must be met the next year itself. NOx and particulate matter (PM) standards must be particularly focussed while keeping an eye on CO and HC too.
LOW NOx DIESEL COMBUSTION
HCCI, pHCCI, UNIBUS, PREDIC, MK, NADI, ACCP, HCLI, HCDC etc are the various low NOx diesel combustion processes. These are low temperature combustion (LTC) processes and they have high EGR rates so that bulk temperature of the cylinder charge can be limited. a boosting/airhandling/ EGR system that effectively provides the right mixture out of boost pressure and EGR rate under all load and speed
conditions is an essential component for maintaining low temperature combustion.
Diesel exhaust after treatment
Lean NOx traps and selective catalytic reduction must be focussed upon. However the SCR has the most potential as it has a higher NOx reduction potential because of its
wider temperature window with high conversion rates
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This paper presents the essential design features of the spark ignition (SI), piston-cylinder engine. Referring to Figure 1 :
S - Stroke
s - Distance between crank and
wrist pin axes
- Crank angle
B - Bore
r – Connecting rod
a – Crankshaft offset
Vc – Clearance volume
Vd – Displacement volume
TDC – Top dead center
BDC – Bottom dead center Figure 1: Piston and Cylinder Geometry of Reciprocating Engine
Definitions
Top Dead Center (TDC) - Maximum travel of piston toward cylinder head. The cylinder volume at TDC is called the clearance volume.
Bottom Dead Center (BDC) - Minimum travel of piston toward crankshaft.
bore (B) - Cylinder diameter (piston diameter = cylinder diameter - clearance).
Stroke (S) - Distance between TDC and BDC.
Displacement (Vd) - volume of cylinder between TDC and BDC.
The Four-Stroke, Air Standard Ideal Otto Cycle
Referring to Figure 2 :
First stroke, Process 6-1 (Induction).
The piston travels from TDC to BDC with the intake valve open and the exhaust valve closed (some valve overlap occurs near the ends of strokes to accommodate the finite time required for valve operation). The temperature of the incoming air is increased 25-35 C over the surrounding air as the air passes through the hot intake manifold.
Second Stroke, Process 1-2 (Compression).
At BDC the intake valve closes. The piston travels to TDC compressing the cylinder contents at constant entropy. Just before TDC, the spark plug fires initiating combustion.
Combustion, Process 2-3.
This process is modeled at constant volume even though combustion requires a finite time in a real engine (cylinder is moving). Peak cycle temperature and pressure occur at state 3.
Third Stroke, Process 3-4 (Expansion or power stroke).
With all valves closed, the piston travels from TDC to BDC. The process is modeled at constant entropy.
Exhaust Blowdown, Process 4-5.
Near the end of the power stroke, the exhaust valve is opened. The resulting pressure differential forces cylinder gases out dropping the pressure to that of the exhaust manifold. The process is modeled at constant volume.
Figure 2: Ideal air-Standard Otto Cycle
Fourth Stroke, Process 5-6.
With the exhaust valve open, the piston travels from BDC to TDC expelling most of the remaining exhaust gases.
Note: The four strokes require two complete revolutions of the crankshaft.
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