DIESEL DROPLET SIZE 1

[harrisj]

Diesel fuel is injected into the combustion chamber as a finely divided aerosol spray. I infer that the better the atomisation, the better the burn characteristics.

My question is: is there an optimum droplet size? If one could atomise the fuel to ever more finely divided mist, would the combustion be more efficient? If one continued to downsize to molecular size particles, it's not an aerosol any more - it's a gas.

So does a diesel ignition work with gaseous fuel? Or is there a nucleation effect where burn commences at some stoichometric point between the lean air in the cylinder and the liquid fuel in the droplet? And how does temperature and burn speed relate to particle size, and vice versa?

My question relates to a super-small diesel engine. I need an injector which delivers tiny amounts of fuel but I'm unsure what the design aim should be.

 

[tbuelna]

harrisj-

Droplet size is characterized by Sauter mean diameter. Sauter mean diameter is the equivalent spherical droplet diameter by surface area per unit volume to the full droplet distribution.

You are correct when you state that lots of very small, widely dispersed droplets is a desireable injection condition. Each individual fuel droplet achieves combustion only at its surface where there is sufficient oxygen present. Fuel/air mixing is achieved through injection spray and intake charge motion. Efficient and rapid combustion requires thorough and complete mixing of fuel and air.

Current DI injector design trend is towards a large number of very small diameter holes, with very high injection pressures. There are practical limits to how small the nozzle holes can be made (typically >.15mm) and to how many holes can be fit into the available space in the tip (usually <9). As for injection pressures, production common rail systems easily achieve 1200 bar and unit injectors can achieve in excess of 1700 bar.

You state that you are working with a "super-small" diesel engine. Achieving precise control with small fuel volume quantities per injection can be very difficult at high injection pressures.

Good Luck.

 

[amorrison]

There are diesel engines for model airplanes and kits to modify "model airplane fuel" engines to diesel.
Google?

 

[GregLocock]

They are compression ignition engines, but use carburettors, and are not true diesel cycle at all.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[SomptingGuy]

HCCI engines can run off pure gaseous fuels and still auto-ignite.