Perkins meet 1977
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'California'regs with new combustion
systemby Graham Montgomerie PERKINS Engines Ltd has— become the first manufacturer to announce that it can meet the proposed 1977 California Air Resources Board legislation with a naturally aspirated direct injection diesel engine. Prior to this many companies have been working along the lines that turbocharging or indirect injection would be necessary especially for NOx emissions but the Peterborough company claims that the new Perkins "Squish Lip" system makes these designs unnecessary.
The problem
Among the various current environmental crises which keep being inflicted upon the long-suffering diesel engine, the problem of exhaust emission has become one of the most important. With a compression ignition engine the pollution problems caused by carbon monoxide and the hydrocarbons are minimal compared with the spark ignition engine but with the oxides of nitrogen (NOx) both engines have similar emission levels. Thus to meet the most stringent legislation in the case of the diesel it is necessary to concentrate on the NOx levels.
The NOx levels could be drastically reduced at source by lowering the combustion temperatures which can be done in several ways. The main options available are: (a) Turbocharging Theoretically this is the simplest way of reducing NOx formation -supply ing large quantities of excess air but without increasing the fuelling levels. This increases the excess oxygen concentration during combustion but also reduces the mean cycle temperatures. Perkins however, claims that the cost impact of such a modification on a volume produced engine is excessive.
(b) Water injection This is an attractive idea on paper but practical con side ra tions make it impossible. Water injection either directly in the cylinder or mixed with the fuel is extremely effective in reducing cycle temperatures because of its high specific heat and latent heat of vapourization. Reductions in NOx concentrations of up to 75 per cent have been achieved with this method. The disadvantages however are legion. For effective NOx control the fuel/ water ratio is in the order of 1:1. This means that a 32 ton unit with a 400-litre fuel tank would need a similar tank for the water! Water also freezes and any normal additive to prevent this increases the hydrocarbon emissions. The fuel consumption is increased and deterioration of the lubricating oil can occur.
(c) Exhaust Gas Recirculation Like water injection this is a good idea on paper but recirculating cooled exhaust gas lowers the available oxygen content of the inlet charge with a corresponding increase in smoke or an equally unacceptable .,duction in smoke limited ower output. Carbon built p in the ring grooves and tcreased oil consumption lso occurs.
Comparing these methods f reducing the NOx conentrations would indicate lat there is a strong case for major swing towards idirect injection systems. Iowever Perkins found that a maintain current power utputs a five per cent icrease in swept volume iould be required as indirect ljection involves an ievitable power loss. This ye per cent increase would ivolve significant changes the overall physical size of ie engine which, again, 'erkins found unacceptble. Thus they took the irect injection system back the drawing board and ad another look.
he "Squish Lip"
At the company's press anference when this ystem as announced Roland ertodo, systems engineerig manager, only told half ie story. Until patent aplications have been finaled details such as injector asition relative to the bowl and nozzle angle were not forthcoming but the combustion bowl shape was released.
Perhaps the most startling piece of information to be divulged was the compression ratio which is 22:1. Mr Bertodo indicated that they would like to try even higher ratios but gasket thickness tolerance and its effect on compression volume becomes too significant. Perkins has insisted on a rotary pump for this engine as it claims that any fuel injection equipment must have a light load advance mechanism for emission regulations and such equipment on in-line pumps tends to be more complicated than their rotary counterparts. The actual combustion process, although not officially revealed, would appear to bear some similarity to the MAN "M" combustion system in that a wall wetting technique is used with the ratio of mixing and fuel evaporation being extremely critical. However, while Mr Bertodo admitted during the question and answer session that the Squish Lip had several similarities he denied that it was the MAN system.
It would appear that, to date at least, the system does not work to any great extent on turbocharged engines but Perkins thinks that this is because "we are using naturally aspirated thoughts on turbocharged engines". One adverse side effect of the Squish Lip is that exhaust temperatures tend to be higher and this will probably mean that exhaust valve seats will be necessary on the Perkins range.
A three hole nozzle is used and its position is critical to within 0.005 in as otherwise the fuel spray misses the lip.
Side effects
As well as comfortably meeting all the pollution regulations, Perkins claims that reductions in noise levels averaging 3dB(A) have been recorded due to the low rates of pressure rise in the cylinder during combustion with a consequent softening of the diesel knock. In addition the lower firing pressure generated means that lighter reciprocating parts can be used and the engine structure in general could benefit by a weight reduction of five to ten per cent. Reduction in top land temperatures of around 150C have also been recorded with squish lip pistons.
It will be interesting to see how the other manufacturers will meet the future emission laws and if they agree with Perkins that it will still be possible with a naturally aspirated direct injection diesel engine. Perkins hopes to be in full production by 1977.
Coinciding with the Squish Lip release, Mr Frank Berry, facilities engineering manager; announced the installation of a complete set of gaseous emission and smoke measuring equipment at the Peterborough works. Costing £40,000, the installation is claimed to meet the requirements of the United States Federal Authorities.