单项选择题 （） the oxygen content in the exhaust gas, （） the combustion is.

单项选择题 Injectors on modern engines are cooled by a combination of the intensive bore cooling in the cylinder by a combination of the intensive bore pockets and by the fuel which is re-circulated through the injector when the follower is on the （）of the cam or when the engine is stopped.

单项选择题 Fuels of different qualities may require advancing or retarding the injection timing, in addition to which if the injection timing is advanced when the engine is running at loads below the （）, then a saving in fuel can be achieved.