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 Combustion 1
kfz-tech.de/PVe61
The age-old question, especially with gasoline engines, is whether there is enough air, fuel, and ignition. The former can actually only be lacking due to blockages or valves not opening (at the
right time). Once the most serious problems have been corrected, there's not much standing in the way of an engine starting.
Fuel is the least of the problems. You don't even need to check the tank, pump, and lines; you can simply add flammable material directly to the intake air, or, in the worst case, starter fluid.
The ignition system has always been critical. The spark plug connector(s) can be removed in no time, replaced with reserve spark plugs, and held against ground, ideally with sufficiently
insulated hands.
But is the visible ignition spark strong enough in the much higher compression combustion chamber? And is the time at which it was generated even correct? Normally, the engine would show
slight signs of life if it were only due to the timing? And only now does the mechanical system become a suspect, and further investigations, for example, into the existing final compression
pressure, are required.
Even if an engine starts well and reliably, that doesn't necessarily guarantee smooth and efficient engine running across all operating ranges. If you want to know what the outcome is, you
should first look at the ingredients. You'll often refer to these as 'fuel,' but this is by no means sufficient, even if you know how to distinguish between gasoline and diesel.
Of the two, diesel has the higher density, and biodiesel, for example, even more so. The same applies to viscosity. The situation is completely reversed when it comes to ease of ignition: (bio)
diesel with a cetane number of at least 51 is high, while gasoline with an octane number of 95 or 98 is significantly lower. However, the required and actually more meaningful motor octane
number is another 10 points lower.
| Cetane number -> measure of ignitability |
| Octane number -> measure of unignitability |
Gasoline now contains 5 to 10 percent ethanol in the engine, and in extreme cases, even 85 percent. Diesel, which meanwhile contains 7 percent biodiesel, is worse for the environment, with a
sulfur content of up to 10 mg/kg. Both ethanol and biodiesel are actually substitutes that can be produced renewably. Sulfur is simply fossil ballast, which a refinery obviously has to put in a lot
of effort to remove.
The energy contained in the fuel, i.e., the calorific value, is the same for gasoline and diesel, with a slight advantage for gasoline. However, combined with its higher density, a liter of diesel is
said to contain approximately 13 percent more energy. The energy content of vegetable oil or biodiesel is 15 percent lower. With ethanol, it is only 40 percent of the gasoline. LPG and natural
gas are not filled up by the liter.
Even if some people have difficulties with this, the distinction between gasoline and diesel engines is feasible and easy to make based on the type of ignition. Although it is possible that one
and the same engine could work according to both principles at some point, it seems impossible in principle. If combustion can only be maintained by electric ignition, it is a gasoline or petrol
engine, otherwise a diesel engine.
The type of injection has long since ceased to be a criterion; perhaps the maximum injection pressure occurring under certain operating conditions still is. The level of geometric compression
is also not a reliable factor in making the distinction. Engines from both directions have already met at 14:0, with turbocharging determining the rest. Only the injection timing could be useful
here, but only if it occurs well before the actual ignition in gasoline engines.
There is therefore a big difference between petrol being injected directly into a combustion chamber and diesel fuel, although in some cases, particularly in ultra-modern petrol engines,
indirect injection is also used. Let's look at the jet emanating from the injector. In a diesel engine, it burns at the edges even before it reaches its target, accompanied by air movement.
None of this applies to gasoline engines. While the fuel in a diesel engine is always fed into a lean or air-rich world, the combustion chamber in a gasoline engine is calculated to be more
harmonious, i.e. aligned to lambda = 1. And even if stratified charge is used, the fuel spray is directed to the richest areas.
Strange, and yet the design must ensure that exactly no combustion takes place. 'Everything obeys my command,' says the engine management system, insisting on determining the correct
timing alone. This is no different with diesel, although it has long been history during injection, as this is precisely what determines the ignition point. The two systems are so close, yet so far
apart.
A longer time for mixture formation, not the case with diesel. The result: Despite calculating a very lean overall distribution, a rich mixture can develop within the burning spray. Unfortunately,
this promotes soot and particle formation. Swirl and the type of injection, namely, a small amount at once in several portions, atomized very finely under very high pressure, can help.
Injection pressures are stated to be well over 2,000 bar, but it's forgotten that in a few specific operating ranges, these can fall below gasoline engine levels from currently a maximum of 350
bar. Otherwise, at its lower end, a CR injector is nothing more than a multi-hole nozzle known from the past, even if it didn't yet have multiple injections.
If, in addition to many piezoelectric layers, a solenoid valve can also achieve this, the conditions in the combustion chamber are comparable. What is created during combustion is called raw
emissions. And hopefully by sometimes being little more than the size of a pinhead, avoids larger particle accumulations.
Yes, you read that right, some combustion is taking place. But the jets are actually supposed to hit the larger surface area in the piston combustion chamber, which only diesel engines have.
There, they are supposed to burn in layers. The question remains, how much fuel is left with such small quantities and immediate combustion?
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