In-cylinder radiation heat transfer in a small direct injection diesel engine

In-cylinder radiation heat transfer in a small direct injection diesel engine

Diesel engine downsizing is of current interest because of the infinite nature of crude oil reserves and the high efficiency of the diesel cycle. Scaling and downsizing
studies can make substantial contributions to the optimization of small diesel engine designs but the scaling of heat transfer losses has not been the focus of recent studies. Radiation heat transfer can represent a significant fraction of the total in-cylinder heat transfer of the diesel engine and hence, developments in radiation heat transfer scaling can have an impact on the success of the overall diesel engine scaling efforts. The present work contributes new radiation heat transfer data from a small direct injection diesel engine and investigates the relationship between radiation heat transfer and engine size. A radiation heat flux probe was developed based on
the optical two-colour method. It was used to measure instantaneous radiation heat flux and the associated parameters KL and apparent flame temperature for
a 0.21 L direct-injection diesel engine at 4 operating conditions giving indicated power values between 1.0 and 2.1 kW. A convective heat flux probe was also developed and used in the same engine for the measurement of instantaneous
convective heat flux at these same operating conditions. In the radiation heat flux measurements, an approach based on post-run calibration of the probe in its sooted state was adopted to compensate for the problem of signal attenuation by probe sooting. In the convection heat flux measurements, the sooting problem on the thermocouple probe was compensated by deduction of the soot layer thickness through the tuning of results during compression to match the motoring results obtained with a soot-free probe.
The radiation heat flux results from the present work are compared with results from other works which have used engines of a larger scale and the present radiation heat
flux data are at the lower end of the overall range. The peak radiation values obtained in the present work are in the range of 0.14 to 0.38MW/m2 which is about half the magnitude obtained with the nearest engine size reported in the available literature. These results point to the significant influence of the soot cloud volume in the radiation heat flux losses. The convective heat flux values obtained in this work are in the same range of values identified from other published works. However, relative to the existing literature, the present work has produced the smallest values for the ratio of radiation heat flux to convective heat flux in terms of both peak and time-averaged
values. This result also reflects the significant influence of engine size on radiation heat flux.
A radiation heat flux scaling analysis was performed based on the time-averaged and peak radiation heat flux experimental data from the present work and other
published literature. The indicated power per unit piston area, IP=Ap has been adopted as a scaling parameter. The time-averaged data appears to be linearly related to IP=Ap while the peak radiation heat flux appears to scale via a power law relationship with an index of 0.83. The linearity of the time-averaged radiation heat flux scaling implies that the time-averaged radiation heat flux is strongly influenced by same size effects while the peak radiation heat flux results are also influenced by emissions originating from soot particles near the surface of the
flame. A new empirical radiation model based on available KL data from various sources including the present work has been developed and implemented in a thermodynamic engine simulation tool. Results from these simulations support the notion that both time-averaged and peak radiation heat
flux are influenced by flame scale but that the peak radiation results are more strongly affected by emissions
from soot particles near the flame surface.