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Natural gas has been the subject of growing interest as a low emissions alternative to conventional automotive engine fuels. The development of a control system for a very low emissions heavy-duty natural gas engine is described. The engine is intended for city bus applications, with emissions targets set well within US 1994 levels. The engine uses a stoichiometric air-fuel mixture with exhaust gas recirculation and a three-way catalyst. The control system was implemented on a prototype hardware architecture designed to facilitate algorithm development. The control system software was constructed from a number of fundamental modules. Good steady-state and transient air-fuel ratio control was particularly important for maintaining optimum catalyst efficiency and hence minimum emissions. To achieve this, the air-fuel ratio control system used solenoid gas injectors and lambda feedback.

Seven hydrocarbons were determined continuously in gasoline engine exhaust over the US FTP cycle using Fourier Transform Infra Red Spectroscopy (FTIR). Three of the hydrocarbons were also determined using Chemical Ionisation-Mass Spectrometry (CI-MS). Tests with and without a three way catalyst illustrated the strong dependence of catalyst performance on the composition of the hydrocarbons emitted. For example, the FTIR indicated that over a cold start FTP cycle, the catalyst took 98 seconds to achieve a 50% conversion rate for ethyne, 183 seconds for ethene and 229 seconds for ethane. The work indicates that FTIR in particular is an appropriate technique for the monitoring of exhaust catalyst performance.

A system for stratifying recycled exhaust gas (EGR) in order to substantially increase dilution tolerance has been applied to a single cylinder manifold injected pent-roof four-valve gasoline engine. This system has been given the generic name Combustion Control by Vortex Stratification (CCVS). Preliminary research has shown that greatly improved fuel consumption is achievable at stoichiometric conditions compared to a conventional version of the same engine whilst retaining ULEV NOx levels. Simultaneously the combustion system has shown inherently low HC emissions compared to homogeneous EGR engines. A production viable variable air motion system has also been assessed which increases the effectiveness of the stratification whilst allowing full load refinement and retaining high performance.