Environmental Engineering Reference
In-Depth Information
Table 9.5 Impact of operation mode on fuel consumption and emissions
Operation modes
Physical properties
Mixture preparation
Stricter requirements for environment protection concerning the exhaust gas limits require improved systems for the preparation of
the air and fuel mixture [ 16 ]. Selective preparation of mixtures for each cylinder of the engine makes it possible to adjust the
optimal mixture in all of the cylinders of the engine. The main limiting factors are the load and the number of revolutions per
cylinder. Sequential injection improves lean running and lowers the output of HC, CO, and NO x emissions. The most important
phases are starting, i.e., the warmup period, accelerating, and braking
Mixture regulation
A Lambda control loop regulating the fuel and air mixture offers further improvements in the mixture stability and the exhaust gas
concentration [ 17 ]. A Lambda sensor installed upstream, i.e., in the front of the catalyst, detects the air and fuel ratio and corrects
the amount of fuel for each cylinder via actuators. The system works within a very narrow range around k = 1 in order to achieve
a high conversion ratio on the three-way catalyst
Variable valve
timing
Variable valve timing allows stable idling. Experience has proven that on-board controlled systems effectively contribute to the
stabilization of the combustion, to the reduction of raw emission, and to the increase of the torque in the low and middle range of
number of revolutions [ 18 ]. The five valve system has three intake and two exhaust valves which greatly improves cylinder filling
Internal mixture
creation
Internal mixture formation systems are expensive because they must be extremely durable [ 19 ]. The biggest dangers are deposits in
the injection systems. Besides valve timing, optimal internal mixture formation with direct injection improves the combustion
stability and lowers exhaust gas emissions during idling
Direct injection
Direct injection through compression spraying lowers fuel consumption and exhaust gas emissions compared to the intake runner
injection. Optimal solutions can be achieved with multi-hole nozzles for the distribution of the air and with variation in the
beginning and finishing time for the injection [ 20 ]
Lean running
Lean running means a Lambda number with k 1. This method effectively decreases SFC by up to 15%, but raw emissions of HCs
and CO increase because of the long combustion time with a low flame speed. Stable combustion and low exhaust gas emissions
in lean engines result from the sequential injection of the fuel, the special design of the inlet channel for swirling and tumbling the
burning gases, and the optimal dose of combustion air to multilayered burning zones in the cylinder [ 21 ]
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