Diesel vs. Petrol
“Why can’t you use diesel oil in a petrol engine?” This was the question which
a friend asked me recently, and I thought I’d answer it today. In order to proceed with
the answer however, we first have to look at how these two different engines work.
In a conventional petrol engine (sometimes called an Otto-cycle engine), a
fuel-air mixture is first sucked into the cylinder, via the inlet / intake valve (on the left
in the diagram below), by the downward moving piston. This is called the induction
stroke. The piston is moved downwards by means of the connecting rod, which links
it to the rotating crankshaft, which in its turn is connected to a relatively heavy disc
called the flywheel, which keeps everything moving relatively smoothly. Once the
piston gets to the bottom of its travel, thus completing the induction stroke the inlet
valve shuts. The piston, still driven by the rotating crankshaft and flywheel, now
reverses direction, moving upwards in the cylinder, and compressing the fuel-air
mixture in the process. This is called the compression stroke. When the piston gets to
the top of its stroke, the ignition system generates a spark at the spark plug. This spark
ignites the fuel-air mixture, and causes it to burn rapidly. This burning raises the
pressure within the cylinder, and drives the piston forcefully downwards. This is
called the power stroke. During this stroke, the piston drives the crankshaft and
flywheel, not only replacing all the energy it has extracted thus far, but producing an
excess of mechanical energy, which can be tapped to drive other machinery – be it a
car, a generator, a water pump, or whatever. When the piston gets to the bottom of its
travel, the exhaust valve (on the left in the diagram below) opens, and the piston
moves upwards, once again driven by the rotating crankshaft. As it ascends, the piston
sweeps all the burnt-up gasses out of the cylinder. This is called the exhaust stroke.
Once the piston has got to the top of this stroke, the cylinder will be emptied of all the
burnt fuel, and will be ready to receive a fresh charge of fuel-air mixture. Now, as the
piston will is drawn downwards again, the cycle we just went through will repeat
itself.
Source: http://www.personal.psu.edu/sam5192/blogs/sarah_mazzocchi/assignment4.html
The four strokes we just went through: induction, compression, power,
exhaust, are the same in the four-stroke diesel engine, which is thus also an Otto-cycle
engine (there is also something called the two-stroke engine, as well as a six-stroke
engine both of which I shall write about at some future date).
What then is the difference you might ask. Well, there are two crucial
differences: First, whereas all petrol engines (except for some experimental versions,
like the HCCI engine illustrated below) have spark plugs, diesel engines do not have
them (though some have a thing called a ‘glow-plug’); Second, whereas in almost all
petrol engine designs (whether carburetted or fuel-injected), the fuel is mixed with air
before being drawn into the cylinder, in diesel engines, the fuel is injected directly into
the cylinder (or at most, into a small chamber attached to the main combustion
chamber), after the compression stroke has been completed, and the piston is near the
top of its upward stroke.
Source: http://www.biodieselmagazine.com/article.jsp?article_id=1753
So, how does the fuel ignite in diesel engines? Well, in petrol engines, the
compression ratio (the volumetric amount by which the charge – the fuel-air mixture –
in the cylinder is compressed) is typically between about eight to one, and ten to one.
The charge is squeezed into a volume one eighth to one tenth of its original size. In
fact, the harder you squeeze the charge in the cylinder, the more power you can get out
of it during the power stroke, which is why sports car engines are typically highcompression engines.
However, the more you compress a gas, the hotter it gets. (If you have ever
used a hand operated bicycle pump, you may have noticed that after a few strokes, the
pump barrel can get quite warm. It gets warm because each compressive stroke of the
pump squeezes the air within the pump barrel, thus raising its temperature, not
because of friction within the pump). Anyway, if you compress the petrol-air mixture
excessively (much beyond about ten point five to one), it will ignite spontaneously.
The diesel engine, in fact, uses exactly this principle to ignite the fuel. During
the diesel induction stroke, pure air, rather than a fuel-air mixture, is drawn into the
cylinder. Thus, during the compression stroke, the piston is compressing pure air
within the cylinder. This allows the compression ratio to be much higher, as there is
no risk of premature combustion in the cylinder. In fact, in diesel engines, the
compression ratio is usually between sixteen to one, to twenty to one (though
sometimes it can be even higher).
Then, at the end of the compression stroke, diesel-fuel – in the form of a fine
mist – is injected directly into the cylinder, and combusts spontaneously upon contact
with the hot compressed air enclosed within it. This initiates the power stroke, as the
piston is forced rapidly downwards by the hot, expanding gasses.
Source: http://www.exploroz.com/Vehicle/Technical/DieselEngines.aspx
So, why not just inject petrol in the same way, directly into the cylinder? Well,
this has to do mostly with the burning characteristics of petrol: being a lighter, smaller
oil molecule, petrol burns far more rapidly than diesel. Thus, direct injection of petrol
typically results in an explosive combustion, rather than a smooth burn. This generally
produces an uneven power stroke. So, although there are experimental direct-injection
petrol engines around, they are not in widespread use.
The slower burn rate of diesel fuel means that diesel engines cannot spin as
fast as petrol engines, as there simply would not be enough time for the fuel to burn
properly if the engine were running too fast. Remember, it takes a brief but still finite
amount of time for the fuel in a cylinder to burn, and if the engine is running very fast
(at a high rate of revolutions per minute), the combustion stroke will be over, and the
exhaust valve will be open before the fuel has been fully combusted. However, this
slower engine speed does mean that diesels are much cleaner burning than petrol
engines, since the contents of the cylinder can be more fully burnt. This means that
diesel engines produce less carbon monoxide than petrol engines. Furthermore, they
are more efficient than petrol engines (they convert more of the energy contained in
the fuel into mechanical energy, and can be 30% more fuel-efficient). Consequently,
diesel engines can do more work per litre of fuel than petrol engines, which accounts
for the far higher mileage per litre for a diesel car as compared to a petrol one. Finally,
under partial loads, a diesel engine’s efficiency is still constant, whilst the efficiency
of a petrol engine under partial load is not very high at all. This means that in typical
driving city conditions, where the engine is idling a lot, diesels are more efficient.
Also, because of the much higher compression ratios involved (20:1 versus 10:1),
diesels have to be built tougher. This often translates into a more maintenance-free
machine. In addition, the fact that there is no ignition system to worry about makes
them simpler to service. Besides all this, diesel engines generally do not generate as
much waste heat as petrol engines, and so do not need to have as much cooling water
or as large radiators installed as their petrol counterparts. One would think this should
make for a lighter engine, but because the compression ratios involved in a diesel are
higher, the engines have to be more robust, and thus there is no real weight saving.
In future posts, I’ll look at other types of engines.