That's too vague, plus I don't know much about diesel engines...
The concept is simple and it's the same way a human drives a car... It's called feedback. You constantly monitor the speed and if it too slow you increase the speed (maybe the voltage or PWM) and if the speed is too fast you slow it down.
...That can actually get tricky because it can end-up unstable and always "hunting" for the correct speed.
These work instead of a gas pedal like in the car with an electric coil that works from 0V to 6V
I'm pretty sure you control the speed by controlling the fuel flow just like a gasoline engine.
A "coil" is not mechanical. Diesels have a glow plug instead of a spark plug and it's probably formed as a coil of wire (like a light bulb with no glass). (I had a toy car with a simple 2-stroke diesel engine, the same as a model airplane engine, and the glow plug was a coil.)
It might use a servo motor instead of a direct-mechanical connection to the gas pedal. Servos use pulse timing to control the angle of the motor shaft. There is a servo library for the Arduino. If you have a servo that runs from a 5V control pulse you can easily control it with the Arduino. I think most modern cars use a servo and cruse control always has a servo.
with an electric coil that works from 0V to 6V
It could be a DC motor or proportional solenoid, but I'm just guessing... The Arduino does not have a true-analog output. analogWrite() is actually PWM which will work to control the speed of a DC motor or the apparent brightness of an LED. You can "boost" the PWM with a MOSFET if you have 6V available.
with this comes another problem the sensor gives a value of 50v peak to peak how can I connect it to my arduino?
A 10:1 [u]voltage divider[/u] (2 resistors) will divide-down the voltage. The resistor values aren't too critical (the ratio is important) but resistors that sum-up to about 10K are usually about right for the Arduino. It's also a good idea to add a couple of [u]protection diodes[/u] in case there are positive or negative voltage spikes.
Is that an analog voltage, proportional to the speed?
for my internship
And internship for what? What are your skills/knowledge? Are you a diesel mechanic? An electronics engineer? Mechanical engineer? Programmer? Etc.?