Emission issues
EVER since small yacht diesels became widely available in the seventies there has been debate over indirect and direct injection. Yanmar favoured indirect injection whereas Volvo Penta preferred direct injection, because with the lower compression ratios around 16:1 compared to 23:1 its engines were easier to hand start using decompression levers.
The small Volvo Penta engines, such as the 7.5 horsepower MD5A, developed maximum power at 2500 revolutions per minute whereas the Yanmar 8 revved to 3200.
INDIRECT INJECTION
Yanmar pioneered indirect injection in its small marine diesels, using what was known as the swirl pre-combustion chamber. Air delivery to the engines was split 50:50 with 50 per cent going to the pre-combustion chamber and 50% to the cylinder. Fuel was injected into the pre-combustion chamber for a thorough air/fuel vaporisation before this mix passed through a narrow throat into the cylinder.
The result was lower emissions than the old direct injection diesels, which emitted relatively high levels of particulate matter, the main pollution issue with diesels. Because this fuel was burnt more thoroughly, indirect injection engines overall emitted less black exhaust smoke; plus less wheezing from exhaust in following seas and were less likely to suffer cylinder bore glazing after being run for
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