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One of the most frightening phenomenon for operators of diesel

engines is the crankcase explosion. Although many

force-lubricated, totally enclosed, reciprocating machines like

diesel engines, steam engines, air compressors and many others

had been running for many hours without any such problems, still

the problem of crankcase explosion is ever present and we cannot

forget about it.

In order to understand the phenomenon of crankcase explosion, we

have to understand the nature of fire, because, after all, an

explosion is a very rapid propagation of fire.

A fire will only start whenever three conditions are met: fuel,

oxygen, and heat.

A fire can very well start inside the crankcase of a diesel

engine when the conditions are just right. For all the recorded

occurrences of crankcase explosions, one factor is common, namely

a hot spot. That is the heat source.

Now, let us take a look inside the crankcase. It houses the

crankshaft, connecting rod, piston rod, cylinder liner, piston,

drive gears or chains and the lubrication oil. To prevent

lubrication oil loss, protect persons from the moving mechanical

parts, prevent contamination of the oil, and many other reasons

the crankcase is totally enclosed and separated from the external

environment.

First, let us look at the moving parts. There is the reciprocating

mechanism consisting of the crankshaft, journal bearings, and

connecting rod. If it is a crosshead engine, the crosshead sliding

and reciprocating mechanism. For the trunk type engine, there is

the piston and the small end bearings.

There are also gears or chains to drive the camshaft. For

reversible engines, the reversing drive mechanisms. There is also

the thrust bearings at the end of the crankshaft to take up the

driven load. There may also be cooling oil pipes, or cooling water

pipes sliding inside the crankcase for piston cooling. The piston

rod itself may also be sliding through a gland assembly at the

under piston space.

All the sliding parts can become a hotspot whenever there is

insufficient lubrication to reduce the friction, probably during

starting of the engine or any abnormal interruption in the force

lubrication oil system.

Next, the use of fuel oil in the engine. Dripping fuel injectors

will result in incomplete combustion of the fuel above the piston

top. If the dripping is serious, the liquid fuel can find their way

through the piston rings aided by the high pressure of the

combustion process to eventually find its way into the crankcase.

For trunk type engines, this is a direct path to the crankcase. It

can become a source of low flash point fuel when mixed with the

lubricating oil!

For dripping fuel injectors, it is common to find unburnt carbon that

will cause the piston rings to stick within their grooves, thus

destroying the sealing function of the piston rings. This will not

only cause the fuel to drip through, but can also cause fire from the

combustion space to blow past through the gaps between the piston

rings and the cylinder bore. This can be a source of heat!

There are other conditions favorable to cause an explosion, for

example the quality of the oil and the effects of oxidation after a

long period of use, the generation of oil mist due to agitation of

the oil spray, the property of the oil, etc.

It is impossible to guarantee that any engine will operate for the

whole of its life without, somewhere, at some time, a hot spot

appearing. Therefore it is essential that should a hot spot occur and

if it goes undetected, an explosion will not result.

That could be the subject of another article... Meanwhile procuring

hard-to-find tools is just as important for making engine adjustments.

Many years of working experience in Marine, Facilities, Construction has given the author material for writing e-books and articles related to engineering, and management. Subscribe to facworld ezine. More information at Marine Engineer and M & E Engineer

Where to Find the Best Diesel Car Hire

There can be distinct advantages when driving a diesel car, hiring one when you visit Spain can mean you make great savings on fuel costs. Although Diesel is a little more expensive than petrol at the pumps it can sometimes produce twice as many miles per gallon in a car when compared to petroleum.

Noisy polluting diesel engines of yesteryear are long gone and buried. The days of driving a diesel car and sounding like the latest John Deere or Massey Ferguson tractor as you popped to the shops are over. Today's modern diesel engines are quiet, far less polluting, economical, and produce far more torque than the equivalent petrol model.

The first diesel engine was designed by a German man named Dr. Rudolph Diesel. It was constructed in 1893 but diesel engines didn't really go into mass production until the 1930s. During this period outdated steam engines were replaced by Diesel engines. They were also introduced into large machinery such as tractors, tanks, trucks and heavy plant, basically any vehicle which required a lot of torque and little maintenance. They were also implemented as static diesel generators used for producing electricity in times of emergency.

During the seventies we saw some of the first instances of Diesel engines being implemented into cars. Since then huge developments have been made in the world of diesel engines. They have been refined, turbo charged to avoid flat spots and made super efficient and quieter. If you are used to driving a diesel you will understand all of the benefits and invariably want them in your hire whilst you are away.

A diesel engine works in much the same way as a petrol engine. It is an internal combustion engine; fuel is ignited to move pistons inside cylinders which in turn move a crank which eventually produces a turning motion. This turning force can then be transmitted to the wheels through a series of gears and prop shafts. The biggest difference is that a diesel engine ignites the fuel alternatively. In a petrol driven engine petroleum is mixed with air to create a fine vapour and then ignited by a spark inside the combustion chamber or cylinder.

A four cylinder diesel engine works by forcing air into a cylinder and allowing the piston to compress it. The diesel is squirted into the cylinder at point when the pressure is just before its highest. As the piston moves back up the cylinder air and diesel are pressurised, which in turn causes the mixture to heat up. Just as the piston reaches the top of its movement they get so hot that they combust forcing the piston back down in the block and continuing the movement of the piston rod and cam which eventually turns the wheels of your car. This cycle is said to be named after Nicolaus Otto, a German engineer.

As a diesel engine relies on compression to form heat and in turn ignite the diesel fuel you can imagine there may be problems when starting a cold engine. Diesel engines have a cold start injector which pumps extra diesel fuel into the engine making it easier to start. The problem still is that the heat generated during compression is dissipated into the cold steel block of the engine rather than igniting the diesel. To over come this, engines are fitted with glow coils these are small metal rods which glow warm when fed with electricity. These draw power from the battery pre heating the cylinders which in turns prevents the heat of compression being stolen by a cold steel engine. Today these are generally found on older engines and larger industrial machinery.

Remember that when you place the key in the diesel ignition there is sometimes a delay when you must wait for the coils to heat up and warm the cylinders before you can start the diesel engine. This is usually indicated by a light on the dashboard. The light indicating the coils will go out once the heat in the coils is sufficient, you can then turn the key and start the ignition.

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challenging question in thermodynamics?

One cylinder in the diesel engine of a truck has an initial volume of 600cm^3 . Air is admitted to the cylinder at 35 C and a pressure of 1.0 atm. The piston rod then does 500J of work to rapidly compress the air.
what is the final volume and temperature?

The key word here is "rapidly".

Because the process occurs rapidly, there isn't significant time for heat to flow out the cylinder walls. We can conclude it to be an adiabatic process.

First law of thermodynamics:
Q = ΔU - W

Solve for ΔU:
ΔU = Q - W

Because it is adiabatic, Q = 0.
Because work was done on the system, the given value of work is actually a negative number. We will later plug in a negative value of W.

Thus:
ΔU = -W

Relation of ΔU to change in temperature (assuming air to be calorically perfect):
ΔU = n*c_v*ΔT

Solve for ΔT:
ΔT = ΔU/(n*c_v)

To get c_v, the molar specific heat capacity, use the following (knowing value of R and adiabatic index):
c_v = R/(k - 1)

ΔT = ΔU*(k - 1)/(n*R)

So, our final temperature:
T2 = T1 + ΔT
T2 = T1 + ΔU*(k - 1)/(n*R)
T2 = T1 - W*(k - 1)/(n*R)

And to achieve n*R, use ideal gas law:
P1*V1 = n*R*T1

Solve for n*R:
n*R = P1*V1/T1

Thus:
T2 = T1*(1 - W*(k - 1)/(P1*V1))

To find corresponding final volume, ASSUME our adiabatic process is an adiabatic and reversible process (it isn't necessarily adiabatic and reversible, but we don't have the information to prove otherwise).

This means that the formula for work is:
W = (P2*V2 - P1*V1)/(1 - k)

And the relation of state 1 and state 2:
P2*V2^k = P1*V1^k

Solve for P2:
P2 = P1*(V1/V2)^k

Thus:
W = (P1*(V1/V2)^k*V2 - P1*V1)/(1 - k)

Factor:
W = P1*(V2*(V1/V2)^k - V1)/(1 - k)

Simplify the term: V2*(V1/V2)^k
V2*(V1/V2)^k
V1^k*V2/V2^k
V1*V1^(k-1)*V2^(1 - k)
V1*(V2/V1)^(1 - k)

Substitute:
W = P1*(V1*V2^(1 - k) - V1)/(1 - k)

Factor:
W = P1*V1*((V2/V1)^(1 - k) - 1)/(1 - k)

Solve for V2:
W*(1 - k)/(P1*V1) = (V2/V1)^(1 - k)

1 + W*(1 - k)/(P1*V1) = (V2/V1)^(1 - k)

V2/V1 = (1 + W*(1 - k)/(P1*V1))^(1/(1 - k))

V2 = V1*(1 + W*(1 - k)/(P1*V1))^(1/(1 - k))

Summary of equations providing results:
T2 = T1*(1 - W*(k - 1)/(P1*V1))
V2 = V1*(1 + W*(1 - k)/(P1*V1))^(1/(1 - k))

Data (notice unit conversions?):
W:= -500 J; P1:=101325 Pa; V1:=0.0006 m^3; T1:=308.15 K; k:=1.4 (standard value for air);

Results:
T2 = 1322 Kelvin
V2 = 15.74 cm^3

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