A Fundamental Investigation into the Problem of NO Formation in Diesel Engines 750848

In order to supplement the on-going development work aimed at reducing emissions levels, in particular NO, in the exhaust of diesel engines a research programme was initiated to investigate the fundamental nature of NO formation.

A rapid acting sampling valve to obtain gas samples directly from the combustion chamber of a running diesel engine was developed concurrently with a mathematical model for the formation of NO in diesel engines, based on the extended Zeldovich mechanism.

Gas samples were obtained from the following types of diesel engine:

1. i)
   A single spray sector of a large quiescent direct injection combustion chamber
2. ii)
   A deep bowl direct injection combustion chamber employing inlet induced swirl
3. iii)
   A Ricardo Comet V indirect injection combustion system.

The temporal and spacial distribution of NO and the local air fuel ratio were determined in each case.

The model depended heavily on the information provided by the gas samples which, in conjunction with high speed photography of the combustion process, indicated that NO was formed only in gases exposed to high temperature flame and that any NO found in cooler areas of fresh air or weaker mixture appeared there solely by mixing with the hot gases, despite the high oxygen concentration. In addition the delay between the combustion and the appearance of NO is clearly shown and this allowed considerable simplification of the model in that only the reactions involved in NO formation needed to be considered kinetically in order to give good correlation between experimental and theoretically predicted exhaust NO content over most of the engine operating range.

It is concluded that the gas sampling valve proved to be a valuable tool in understanding the fundamentals of NO formation in particular and combustion problems in general and greatly assisted in the formulation of a mathematical model for prediction of NO emissions from diesel engines.

NITRIC OXIDE (NO) is one of the more undesireable pollutant constituents of vehicle exhaust gas emissions because of its connection with the formation of photochemical smog in certain locations throughout the world (1)*

As a consequence of this connection, legislation was proposed to progressively reduce the maximum permitted levels of NO in vehicle exhaust. Maximum levels of the other gaseous pollutants, carbon monoxide (CO) and unburned hydrocarbons (HC) were also proposed in the legislation and it was clear that eventually the relatively clean diesel engine would be unable to meet the proposed emissions levels in its normal form.