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A 1.8 litre four-cylinder engine with a slice between the head and the block carrying instrumented plugs has been used to study the growth of combustion chamber deposits and some of their effects on engine operation. Different techniques for measuring deposit thickness, knock onset and deposit effects on the thermal characteristics of the cylinder have been developed. Deposit growth as measured by deposit weight on the plugs is reasonably repeatable from run to run and cylinder to cylinder. The presence of deposits already in the cylinder does not affect deposit growth on clean plugs introduced into the combustion chamber. Deposit thickness and morphology vary substantially at different locations, the thickness being greatest at the coolest surfaces. Deposits increase the flame speed and reduce the metal temperatures just below the surface. They also reduce the mean heat flux away from the cylinder.

Investigations into fuel compositional effects on emissions using model and full range fuels suggest aromatic components promote NOx conversion over the catalyst Steady state results derived from a single engine (Ricardo Gasoline Fuels Consortium data) show that at a typical part load condition, the catalyst removes NOx less effectively with lower aromatic fuels. Neither CO nor H2 contribute significantly to catalyst performance. Two vehicles were tested over a European cycle. Toluene formed more combustion chamber NOx, offset by increased catalyst conversion efficiency giving lower tailpipe NOx than isooctane in the vehicle with the better catalyst light-off and AFR control.

A “CARS” System using a modeless dye laser has been extensively calibrated and shown to give average temperatures of acceptably good accuracy. It has been used to measure temperatures in the end-gas of a single-cylinder E6 engine under knocking conditions using propane, commercial butane, iso-octane and a primary reference fuel made up of 90% iso-octane and 10% n-heptane by volume. These measurements show that there is significant heating of the end-gas because of pre-flame chemical reactions for all the fuels except propane. Propane has to be compressed to a much higher pressure compared to the other fuels studied in order to make it knock. At a given engine operating condition, there is significant cycle-by-cycle variation in both combustion and knock.

Six full range gasolines were tested in two engines (one with a catalyst) operated at 4 steady states. Engine-out regulated emissions responded to equivalence ratio, Φ, in the accepted manner. For both CO and NOx, there was a characteristic, single emissions response to changes in Φ. Changing fuel composition will primarily alter the production of these emissions by modifying the stoichiometric air/fuel ratio, projecting engine operation onto another part of the Φ response curve. These Φ effects, which are independent of engine design, also determine how operating conditions affect engine-out CO and NOx. Speciated hydrocarbon measurements at engine-out and tail-pipe confirm results seen in previous test-cycle based programmes.

A set of designed fuels was tested in two European gasoline vehicles driven over the standard European Drive Cycle. Both regulated and speciated emissions were measured, together with HC, CO and NOx, pre- and post-catalyst. The main fuel set was a 3 by 3 matrix, where mid-range volatility (T50) and aromatics were independently varied from 85°C to 115°C for T50 and from 25% vol. to 45% vol. for aromatics. Two further fuels, together with the centre point fuel from the main matrix, formed a back-end volatility (T90) subset experiment. The fuels were blended from mixtures of pure chemicals in order that the chemical and physical properties could be closely controlled and kept independent. The findings of this two car trial are generally in line with the recent EPEFE programme and confirm that fuel changes which reduce one type of emissions (HC and CO) generally increase another (NOx) and vice versa.

Vehicle-based studies have shown that a reduction in the aromatic content of gasoline fuels can result in increased NOx emissions from catalyst-equipped vehicles. A study with simulated exhaust gas has shown that light paraffins, especially methane, are unreactive and cause substantial breakthrough of unreacted NO over the catalyst. However, unsaturated exhaust components including aromatics are effective reactants and play a large part in converting NO over the catalyst. Engine tests have shown that methane is predominantly produced by fuel paraffins and olefins, but hardly at all by aromatics. Thus it appears that methane generated during combustion of low aromatics fuels may be the cause, wholly or in part, of the poor NO conversion efficiency observed when catalyst-equipped cars are operated on such fuels. However, it cannot be excluded that low aromatics fuels are associated with increased air-to-fuel ratio which will also contribute to poor NO performance.

Increasing interest in gasoline engine emissions has focused attention on the fuel compositional and emissions effects that govern NOx conversion over the catalyst. This study reports the transient effects of individual species emissions and catalyst conversions on NOx conversion made using Fourier Transform Infra Red (FTIR) spectroscopy of the engine-out and tailpipe emissions (regulated and speciated) during the testing of a catalyst equipped gasoline vehicle run on multi-component model fuels over the standard European cycle. FTIR measurements confirm that transient NO conversion is directly correlated with that of CH4, especially within the Urban Drive Cycle (EUDC). Other hydrocarbon species do not govern the transient variability in NO conversion. This vehicle maintained ϕ≤ 1.0 practically throughout the EUDC and consequently no correlation was seen between transient NO conversion and equivalence ratio.