Search Results

Hydrocarbon wall quenching has been studied using a 19mm diameter, 1m long combustion tube, open at one end. Mixtures of propane, heptane, iso-octane and gasoline, initially quiescent, were burnt with the ignition source at the closed end. The post-flame HC levels were measured at a series of axial locations using a fast FID. The results indicate that the effective quench layer thickness increases significantly as the molecular weight of the fuel is increased. The diffusion/mixing time constant of the quench layer was found to be approximately 0.1s for propane, 0.4s for iso-octane and 1.0s for gasoline. The axial variation of residual HC levels suggests that flame stretch is a factor influencing the extent of the quench layer.

An investigation has been carried out to measure the spatial structure of the turbulence in a model IC engine at near to BDC, inlet stroke. Flying hot wire anemometer measurements have revealed details of the effect of the inlet port angle and valve lift on the spatial turbulence structure. Cycle resolved turbulence intensity and integral length scale are presented at three cutoff frequencies of 10 Hz, 100 Hz, and 1000 Hz. Although the situation is transient, conventional analysis methods are shown to be useful. The inhomogeneity of the intake turbulence is found to be considerable. It is also shown that high turbulence energy content occurs at lower frequencies. The flow also exhibits high cycle-to-cycle variation in the mean velocity although this variation was not sensitive to the cutoff frequency. At low cutoff frequency the integral length scale of BDC turbulence was found to be comparable with the valve lift. The rôle of the inlet jet flow is shown to be crucial.

A thermal imaging system has been developed for viewing and recording the cylinder head surface temperatures of an internal combustion engine. The system consists of an I.R scanner, associated calibration and image processing equipment and an infra-red transmitting window mounted in the piston. The infrared window material used (silicon) has thermal characteristics close to those of a normal piston. The two dimensional temperature distribution of a cylinder head surface has been measured during start-up. The imaging results from the camera were checked against the readings from the thermocouples fitted into the cylinder head. The agreement was very good, and gives confidence in the system.

A fast-FID sampling technique has been developed to study top-land crevice out-gassing from the moving piston of an SI engine. A sampling probe, housed in the piston crown, delivered gas to the FID head via a flexible transfer tube. Comparisons of the HC concentrations at the top-land location and the bulk gas above the piston crown confirm that HC material is out-gassed from the top-land region during the expansion stroke and is followed by more rapid out-gassing after EVO. The removal of wall HCs has been detected during the exhaust stroke from the probable scrolling effect produced by the rising piston scraping unburned material from the cylinder wall.

The action of crevices in an operating SI engine has been studied with a fast-FID. A single-cylinder Ricardo E6 research engine was fuelled with propane and operated at 1300 RPM. FID measurements in the exhaust port have shown that advancing the ignition timing from 30°BTDC (MBT) to 60°BTDC raises the HC concentration by 25% during the first 120°CA of the exhaust stroke and by 20% for the remainder of the stroke. A static “artificial” crevice of known volume, mounted inside the engine cylinder was used to study the differing HC outgassing characteristics at the two ignition timings. When sampling in-cylinder at the mouth of this crevice, the opposite effect of a 50% reduction in outgas HC concentration occurred when the ignition was advanced to 60°BTDC. It is argued that advancing the ignition causes earlier enflamement of the static crevice and induces burned as well as unburned gas to enter the crevice thereby diluting the HCs from this source.

A fast-response FID has been used to study the concentration of hydrocarbon material at four different locations in a firing SI engine. These were: on the flat surface of the cylinder head, at the exhaust valve seat crevice, just downstream of the exhaust seat in the exhaust port and 20mm downstream from the valve stem in the exhaust manifold. A close-fitting sleeve arrangement enabled the sample tube to be positioned accurately flush with the head face and also to be slid away from the wall into the bulk gases whilst maintaining a gas-tight seal. In this way, wall effects could be noted by moving the probe position without stopping the engine and directly comparing with hydrocarbon levels in the bulk gas. Using propane in a fully-warmed up engine, results showed the presence of HCs residing in a 2mm layer adjacent to the wall after EVO and during the exhaust stroke. These could be detected flowing over the valve seat after EVO and were also observed at the manifold location.

Flying Hot-wire anemometer measurements have revealed the effect of the inlet port angle, and valve lift on the spatial structure of turbulence in a model internal combustion engine. The present experiments are carried out at the BDC, intake stroke at a low piston speed in a dynamically similar flow. The data are presented using both cycle resolved and ensemble averaging techniques. The experiments are carried out at three different measurement positions across the engine cylinder. The inhomogeneity of the intake turbulence is found to be considerable. The role of the inlet jet flow is shown to be crucial.