Just a quick refresh of the main fundamental difference between a gasoline SI engine and a diesel CI engine which is, quite obviously, the combustion process.
Within an SI engine, operating under normal conditions, the sequence is: -
Mixture Formation - Combustion Initiation - Kernel Formation - Rapid Combustion - Exhaust
A homogenous charge is formed within the cylinder volume, either through fuel being introduced into the air charge, remotely from the cylinder, or injected directly within the cylinder.
Upon firing the spark plug, a laminar flame spreads from the spark plug electrodes and a roughly spherical flame kernel is formed. This then quickly develops into a turbulent flame, due to in-cylinder gas motion; with this motion increasing the burn rate of the charge. All before the end gases auto ignite and cause Detonation.
Within a Common Rail Direct Injection CI engine, again under normal operating conditions, the sequence is: -
Start Of Injection - Ignition Delay - Premixed Combustion - Mixing Controlled Combustion & EOI - Late Combustion - Exhaust
After the start of injection, directly within the cylinder, a heterogeneous charge is formed by way of a jet of high pressure fuel at the edges of which liquid fuel sits in suspension as droplets, slowly evaporating within the relatively low in-cylinder temps.
Around these droplets sit local pockets of vapour, some of which possess an AFR within the combustible range. As the compression stroke continues the temperature within the cylinder increases and these combustible areas begin to do so, spontaneously. This Premixed Combustion can and does occur simultaneously in many different areas at once and is, in some ways, analogous to detonation in an SI engine, with a rapid rate of pressure change. This is associated with the characteristic Diesel Knock/Combustion Noise & damage to engine internals.
The Premixed Phase develops into the Mixing Controlled Phase; as the fuel continues to be injected the pressures, temps, charge kinetic energy & rates of evaporation all increase. The rate at which the fuel is burnt relies upon the fuel molecules being exposed to O2 in the correct ratio with which to react and the formation of areas of combustible AFR mixture. This is a function of the available O2 (boost pressure & internal EGR) and in-cylinder charge behaviour i.e. rate of evaporation/fuel quality.
So, for a given CI engine & fuel type, to increase the torque produced the mass of fuel injected is the dominant factor and is a function of (Rail Pressure, Injection Duration).
The Injection Duration (SOI to EOI) is always constrained by the following: -
SOI
The piston must be near enough to TDC to allow the injected fuel to be within the bowl of the piston and not impinge on the relatively cold walls of the cylinder – or some of the fuel is wasted.
With this as the maximum, the advance of the position of SOI towards such is also further restricted by an increase in Ignition Delay. This is due to charge temps being too low to promote vaporisation, and therefore combustion, leaving the fuel ‘sitting around’ waiting to be combusted. Which, when it does, tends to occur in one bulk reaction.
EOI
For good mixing & therefore combustion to occur, the EOI must occur before the injection spray misses the piston bowl where gas velocities begin to slow drastically (bearing in mind reduced O2 & increased EGR). Good combustion therefore begins to rapidly diminish, as the piston moves towards BDC. Another constrain on EOI is the increase of exhaust temps due to slow reacting fuel.
Once the above constraints are encountered the only way to achieve more torque is to inject the fuel at a higher pressure with a corresponding increase in boost pressure (available O2 & compression).
More boost pressure means more available O2 and higher compression temps with a decrease of Ignition Delay but an increase in rate of overall pressure rise, a higher peak pressure and a shorter combustion duration – certain provision can be made to offset this by intercooling the boosted charge. Exhaust temps are also reduced (analogous to advancing the spark in an SI engine) as the burn duration is reduced. However, the increase in rate of pressure rise can be, as mentioned above, very damaging to engine internals in the same way that knocking in an SI engine can be.
So, in a nutshell, the challenge is to be able to inject enough fuel between the piston position constraints of SOI & EOI without an excessive Ignition Delay and provide an acceptable rate of pressure increase during Premixed Combustion. Whilst moderating the rate of pressure rise and decreased burn duration during Mixing Controlled Combustion but reducing the high exhaust temps that occur (the Late Combustion) due to the excessive fuelling by increasing boost pressure/reducing charge temps.
And that’s before we start talking about smoke…..
See why I hate Diesels?
MS
ps please attribute any typos etc to the few G&Ts I've imbibed, just to wet the whistle!
