Over the years, manufactures have taken at least three different approaches to reducing pumping losses. The most well known here is lean burn, but cylinder deactivation and atkinson-like valve timing strategies are also utilized. So, which is the best? And is it best to combine strategies like cylinder deactivation and atkinsoning for differing power capabilities?
Here is what I have gathered as far as advantages and disadvantages of these three systems.
Lean burn
Advantages:
- Reduces pumping losses, duh
- Requires no additional mechanical complexity
Disadvantages:
- Vastly increased NOx emissions for certain fuel-air ratios
- Requires specially designed cylinder head, ignition system, and possibly fuel injectors to work properly and avoid engine damage
- Requires additional kitty kats :3 to meet emissions
- Can kick in and out with a lurch
- Limited rpm and load range where engine can operate smoothly
- Limited ambient condition range where engine can operate in lean burn (Too cold? No lean burn for you)
- Requires special port design which can limit high rpm flow
Cylinder deactivation
Advantages:
- Reduces pumping losses AND frictional losses
- Can be utilized during DFCO to reduce engine braking
- No problems with emissions
Disadvantages:
- Requires additional mechanical complexity
- Can kick in and out with a lurch (many owners of honda VCM engines have DISABLED VCM b̶e̶c̶a̶u̶s̶e̶ ̶t̶h̶e̶y̶ ̶a̶r̶e̶ ̶p̶u̶s̶s̶i̶e̶s̶ ̶t̶h̶a̶t̶ ̶c̶a̶n̶'̶t̶ ̶h̶a̶n̶d̶l̶e̶ ̶a̶ ̶l̶i̶t̶t̶l̶e̶ ̶b̶u̶m̶p̶ because they don't like the lurch)
- Can cause unusual engine wear patterns if the system does not alternate between cylinders to be deactivated.
Atkinson valve timing
Advantages:
- Reduces pumping losses
- Can utilize an existing technology already present on most cars (cam phasing)
- No emissions problems
- Can vary effect widely based on engine load and rpm (not as restrictive range of use)
- No lurching
Disadvantages:
- Kills torque if cam phasing is not utilized (prius engine)
- Requires a cam phaser with a very wide traverse capability to switch between lots of atkinsoning and high rpm power
- Requires engines to be DOHC
- Cannot be utilized on single cylinder engines and engines that do not share the same intake manifold/airbox(Not a problem for most car engines)
Increased EGR flow/Cooled EGR
Advantages:
- Reduced pumping losses
- Reduces NOx emissions
- Utilizes a system already in place on most modern car engines
Disadvantages:
- Clogs intake faster (resulting in reduced power and gas mileage)
- Limited engine load conditions available for using high volume EGR (Can cause slow combustion and stumbling)
- EGR cooler required for high volume EGR flow
So far atkinsoning seems like the best choice, I wonder why Honda is still playing with VCM. Mazda gets the picture, and has implemented atkinson valve timing capabilities in its new skyactiv engines. I plugged my scangauge into my mom's CX-5 and noted that under normal acceleration the throttle plate is WIDE OPEN! Atkinson for the win!
Here are some pictures of a VCM engine, showing one of its disadvantages:
Here is the working cylinder head
And this is the VCM head, looks alot cleaner doesn't it?
On the top of the cylinder heads, its the complete opposite. The working head is clean, and the VCM head is dirty. I would assume this has to do with oil not being as hot on the VCM head and caking up.