Controlled Auto Ignition - Lean Mixture, Why?
Controlled Auto Ignition - Lean Mixture, Why?
(OP)
Hi everybody,
I am starting to investigate controlled auto-ignition, also known as HCCI.
All related literature claims that CAI operation can only be achieved with high lambda. I have understood that you need a good homogenious mixture, so ignition is distributed evenly, but why is this not possible with more fuel (also distributed evenly)? Unfortunately I can not find an explanation why this is the case.
Could anybody help me?
Thanks,
Chris
I am starting to investigate controlled auto-ignition, also known as HCCI.
All related literature claims that CAI operation can only be achieved with high lambda. I have understood that you need a good homogenious mixture, so ignition is distributed evenly, but why is this not possible with more fuel (also distributed evenly)? Unfortunately I can not find an explanation why this is the case.
Could anybody help me?
Thanks,
Chris





RE: Controlled Auto Ignition - Lean Mixture, Why?
One problem is that the rate of pressure rise becomes explosive. A tough problem with HCCI is controlling the rate of heat release. They seem to be doing this by using high EGR and "not quite homogeneous" operation, but that only works at lighter engine load. If you have high enough cylinder pressure to get autoignition with minimal EGR at higher engine load, the combustion starts to resemble a massive detonation in the cylinder.
True HCCI with early fuel injection is a nightmare to control. The temperature at end of compression affects the moment of ignition, but the temperature at end of compression is different if the outside temp is -30 C compared to if it is +30 C. At light engine load you can play with the EGR (and whether you cool it or not) to affect this. Near full load, what do you do to control the ignition timing?
I don't know if "true" HCCI will ever see production except maybe in very narrow speed/load regimes on engines that are otherwise direct-injected gasoline or diesel. Production diesel engines seem to be starting to implement lessons learned from this research, but they seem to be using "more-homogeneous but not truly homogeneous" mixture preparation. The new low-emission diesels seem to be using lower compression ratios (i.e. longer ignition delay), and really high injection pressure (to mix air and fuel faster), and EGR systems in which the EGR can be cooled or not, and some of them are using a low-pressure EGR drawn from after the particulate filter. It seems that some of them are using *late* injection timing and relying on the ignition delay to serve as the pre-mixing period.
RE: Controlled Auto Ignition - Lean Mixture, Why?
First is, NOx reduction is usually a driver for HCCI combustion solutions. This can only be achieved with an ultra lean mixture (due to the cooling effect of charge dilution)
Second is, detonation is a violent event. Existing commercial engine structures are incapable of withstanding ongoing detonation of a stoichiometric mixture. Today's HCCI combustion recipies are lean enough to be within existing engines' structural and material limits.
RE: Controlled Auto Ignition - Lean Mixture, Why?
RE: Controlled Auto Ignition - Lean Mixture, Why?
HCCI or CAI combustion is essentially "controlled" detonation. A stoichiometric, homogenous intake charge (ie: an engine at full load) would auto-ignite well before TDC in a fixed CR piston engine (using a CR otherwise suitable for HCCI) causing a very steep pressure rise and high mechanical loads. A very lean mixture is less susceptible to auto-ignition, but unfortunately also produces less power. HCCI combustion chemical kinetics are incredibly complex, and as a result HCCI combustion is very difficult to control reliably.
The typical methods employed to control HCCI combustion are A/F ratio, charge temperature, EGR rate, variable valve timing and variable compression ratio. These are all expensive options for a production engine, but the benefits are reduced part throttle fuel consumption and significantly reduced NOx emissions.
No one has yet gotten an HCCI engine to function reliably outside of a laboratory environment. There are still huge issues keeping it out of the marketplace, such as making sure it works reliably under all operating conditions for the life of the vehicle, and reducing or eliminating its nasty NVH characteristics. Even though there are lots of auto companies, spending millions of R&D dollars to develop it, I don't see it making its way into the marketplace anytime soon. The development challenges are still too great.