Jump to content
Mechanical Engineering


Recommended Comments

In petrol the flah and fire point is High when compared with Diesel.

If we gone through the colorific value of the petrol it is very very less it quickly burn and quickly decompose. But in case of Diesel it will catch fire less slow than petol and stand still in burning and evolving more power. For that purpose the more power requirement we are using Diesel Engines.

Link to comment

The Temperature required for combustion is higher for diesel and the spark plug is not enough to produce the temperature.

Even, if it is used, it causes pre-ignition and knocking, which is not preferable.

That's why it has higher compression ratio of 14-22, which is pretty much compared to petrol engines of compression ratio 5-8.

Thus, there is no point of using spark plugs in not only diesel engines but also in CI engines,which may use biodiesel, kerosene, etc.

Link to comment

Oh god...

I guess diesel engines don't use spark plugs because the air/fuel mixture isn't as explosive as is in the case of petrol (or gasoline). Therefore you cannot spark ignite it. That is the reason why diesel engines have a high compression ratio (close to 20 as has been said), which heats up the air being compressed in the cylinder to a high enough temperature. Fuel is then injected straight into the cylinder, where it ignites because of the high temperature of the air and that is it. That's why spark plugs are not needed.

The thing is, in petrol engines you used to squirt the fuel into the air intakes, where it mixed with the incoming air and filled the cylinders. If you used compression ratios as high as diesels, you would get premature explosions, i.e. knocking. That's why the ratios are lower and that's why you need a spark plug to ignite it.

Directly comparing the two engines at the same compression ratio, gasoline engines are more efficient (at least at ratios of around 10 to 15). It would be possible to make a gasoline engine with a ratio of 15 these days, since we have mastered direct injection in petrol engines as well, but that would stress the heads (and pistons) a whole lot more - petrol mixture burns at ~1200°C, while diesel does at ~800°C - a big difference. Plus the pressures would be very high, etc. It is possible in theory, but is not practical (emissions would be a b*tch as well, i think higher cylinder temps cause more NOx to form in the exhaust gases).

EDIT: that's how i see it, but i could be wrong. Correct me if i am.

Link to comment

Porque los motores a diesel su combustión la hacen con incremento de temperatura por presión del pistón debido al Poder Calorífico del combustible. Es PC es mayor el diesel que la gasolina.

Link to comment

Diesel engines don't require a spark plug, because they rely on high compression. I like to use the example of holding a bicycle tyre pump over the flat of your hand, and pumping it hard. The burning sensation on your skin is the answer. High compression of air = heat. Add atomised diesel fuel and you have ignition. 

Link to comment

air inside cylinder is compresssed by piston to a very high pressure, thus the air abruptly become very hot and its temperature increased from 35 to 1000 degree centigrade equivalent or more than  the temp given off by spark plug. therefore hot air, atomized fuel mixed and exploded spontaneously without the use of spark plug.


Link to comment

The "blue collar" explanation is simple: higher compression of the Diesel engine negates the need for an 'ignition system'. Keep in mind the mechanical principals: compressing atmospheric gasses, 2 to 3 times higher than petrol engines, creates a high temp micro climate then injecting atomized fuel creates the rapid burn and expansion. A Diesel engine can utilize a wide variety of fuels - Rudolph Diesel's intention by invention was to utilize local (vegetable base) oils as a fuel source. As far as "glow plugs": these are a starting aid to preheat combustion air prior to the engine achieving operating temp. **The diagram attached to this discussion is depicting a "two stroke" diesel.


Link to comment

It's ok, high compression and heat, but what happens chemicaly with diesel at high pressure and temp, i think that's the point. That can explain who knows thermodynamics well.

Link to comment

One thing we know. We can’t use fuel in liquid form during compression, because liquids are incompressible. Now we have option to use diesel in vapour form. At atmospheric pressure, boiling point of diesel is 250°c to 300°c. And diesel in fuel tank have temperature around 40°c to 50°c. So it is not possible to make vapors (as in carburetor of petrol engine).
So, how can we convert diesel into vapour form?
There are three options:
            1.         If we put liquid diesel into a medium having pressure less than vapour pressure of diesel, it means we create vacuum inside the cylinder. During suction stroke we have vacuum and we can convert the diesel into vapors, but during compression stroke we must avoid auto ignition due to high compression ratio. The one and the only way to avoid the auto ignition in such case is to reduce the compression ratio.  Which is not possible (2nd law of thermodynamics, higher is the pressure more is the power) if we want to get power from engine.  Another important reason for using higher compression ratio is to get higher expansion ratio (more piston displacement) thus more power with same fuel consumption.
Work done = pressure (mean effective pressure) × piston cross-sectional area × piston displacement.

But still if someone manages this miracle, how he overcomes condensation of diesel during compression stroke where the pressure goes up again. So drop this option.

            2.         Increase the temperature of liquid diesel before putting into engine cylinder which is possible but not reliable. We have to use pre-heaters, which cost energy consumption and makes vehicles bulky.

            3.         Put liquid diesel into high temperature air (greater than saturation temperature of diesel) and provide some time for heat exchanging between them.

In practical air standard engines third option is perfect as we produce high temperatures by compression. But still we can't use spark plug for ignition, since mixture present in the cylinder is heterogeneous (somewhere mixture within ignition limits and somewhere not). Ignition by spark plug required homogenous air fuel mixture. Now we have only 1 option to ignite the diesel, that is; compresses the air and raised the temperature of air up to ignition temperature of diesel and inject the diesel in cylinder.

Link to comment

Add a comment...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Join Mechanical Engineering network

    Join us (login) to get full access : Please sign up to connect and participate.

    To download files...please login

  • Create New...