With both valves closed the piston comes up on the compression stroke super heating the air 18x ambient pressure. The injectors then spray atomized fuel into the combustion chamber where it's ignited by the super heated air.
With all that said, and correct me if I'm wrong, what provides the timing of the injectors? Is it at TDC or before, like a gas engine? Is the camshaft involved in deciding when to actuate the injectors?
What is the flash point of diesel fuel? How hot does the compressed air get?
Inquiring minds want to know.
Mark
Let's get the cycle correct first. The compression ratio may be 18 but that's a volume ratio and not a pressure ratio.The pressure ratio is more like C x 18^1.4 where C is a contant that allows for the fact that the valves aren't completely closed throught the compression stroke and it's value is around 0.65. (It varies with the valve timing so it's different for each engine.) That gives a pressure ratio of 37.2 , not 18. ( If the ambient pressure is 14.7 psia, then the compression pressure is ~ 514 psia.)The temperature ratio is ~2.8 for this situation and that gives you an air temperature of ~996°F (( 2.8 x 520°R)-460°R)on a 60°F day.
The fuel autoignition temperature is ~425°F
In order to inject the fuel and break it up into a mist, the injector pressure must be well above the compression pressures through out the injection process.
Because the combustion reaction takes some time to start (ignition delay) and the fuel injection is not instantaneous, the introduction of fuel must take place in advance of the piston reaching TDC. The advance is taken care of by the fuel injection system. Most diesels are timed around 19-23° BTDC, as I recall. My NH has a ****** device that senses coolant temperature and retards the timing if the coolant is less than 120°F to reduce some emmission component.
There are several ways to control the injection process;mechanically or electroncally. The mechanical methods use the pump it self or a rack system where the thottle rotates the injectors to set the fuel charge and a pushod mechanically causes the injection. ( IIRC, some inline injection pumps use racks.)
Most of the modern tractors use a metering sytem in injection pump to pressurze and meter the fuel delivery and the pump timing gears are driven by the crankshaft as is the cam for the valves. Everything is a hard mechanical connection.
In the electronic systems, the timing is sensed from the crankshaft, the load is sensed by a PCM (power control module), the timing and the quantity of fuel injected are controlled by an electrically controlled fuel injector driven by the PCM and the muscle for the injection comes from engine lube oil via a high pressure pump (Ford Powerstroke System)
I hope you find this helpful.
