For all you Turbo fans and Naturally Aspirated Fans,
Problem:
One of the main reason high revving engines has a little more challenge meeting tighter emission requirement is that a higher range of operating RPM requires more precise control for clean combustion and this is limited by valve timing and lift.
The key variable is that fuel largely burns at the same rate regardless of rpm. During a combustion cycle, you have intake, compression, expansion, and exhaust stroke going on for each cycle. The ignition timing, valve timing and valve lift needs to be optimized for each specific rpm so that during the expansion stroke, the fuel is burned as much as possible before the exhaust valves open. As the rpm is increased, the pistons are moving faster but the burn rate of the fuel is fixed. Therefore, fuel may not be entirely burned at the end of the expansion stroke prior to exhaust valve opening. As a result, unburned fuel goes out the exhaust. Also at high rpm and high load, the risk of detonation can blow engines so engines are setup to run rich at WOT (Wide open Throttle).
The good news is that Ignition timing is varied and adjusts to RPM and load. This has been the case on moden engines for a long time, so ignition timing is already been optimized for each RPM. However, valve timing and lift is
not infinitely adjustable and has limititations. The variable lift in VTEC does give you adjustment for lift and timing but it is a stepped ladder increase. For the current VTEC system in S2000 and NSX, there are only 2 modes. Therefore only 2 sets of cam calibrations. In a perfect sense, the 2 cam calibration are optimized for only 2 rpms but is not optimized across all rpms. On the other hand, the VTC system on iVTEC goes a longer way because it can vary timing over a larger range. But I'm not sure it can respond fast enough with quick changes to RPM. The VTC in ivtec only varies timing but not lift and is currently only employed on the intake cams.
Solution
One approach to running cleaner is to optimize the valve timing and lift system for every rpm and engine load condition. I believe AVTEC was a step in the right direction in achieving infinitely variable lift and cam timing. But it is currently scrapped due to concerns with excessive lash and potentially too much wear. With the current iVTEC system, an improvement can be obtained by adding VTC on both the intake
and exhaust cams.
BMW’s valvetronis achieves variable timing and lift but it has excessive weight and inertia issues to prevent it from high RPM applications. Hence most past M series model do not employ this technology.
In addition,
laser ignition system would also help with cleaner combustion by having a stronger flame initiation. Ignition timing could also be improved because laser igniters can pulse within nanoseconds instead of milliseconds with spark plugs. Laser can also burn lean mixtures so there would be no need to run rich and pollute during WOT conditions. Lasers could focus their beams into the middle of the column, from which point the explosion would expand more symmetrically. Refererence to this link.
http://www.gizmag.com/laser-ignition-spark-plug-alternative/18469/
Why Turbos?
More power per unit of displacement. Engines can be smaller with less cylinders. Less cylinders = less frictional loss with less cylinders and valve train to move. More power is relatively easy, just crank up the boost. Why use crazy rpms to chase horsepower when OEMs can just dial up the boost to meet power goals. With high rpms, you are back to emission issues and more mechanical friction. Turbo is a more efficient solution, no doubt. The power is not totally free because using exhaust gas to spin turbines creates back pressure on the exhaust stroke so it does choke the engine a bit. In addition, engine components will need to be reinforced for the higher output, which you already know that.
Innovation?
The question is, is Turbo innovative? Turbo has been around a very long time. It is a proven solution to meet current/future green mandates. Because the technology has been around, it is easy for OEMs to quickly adopt it without an excessive amount of R&D to invent new solutions. For this reason, many companies have jumped on this bandwagon. It is a cost effective solution to simultaneously meet green mandates and satisfy power hungry drivers.
Will Turbo be the way of the future? Let’s use history as a guide. In the 70 and 80s we saw turbos come into play. The OEMs needed to compensate for smaller engines and lower compression so they added Turbo to give us the best of both worlds. In theory is was the ideal solution but in practice, not much so because of problems with heat and lag. However, in the 90s turbos phased out as innoviation phased in with electronics controlling emissions for precise fuel metering. Big engines came roaring back. Innovation drove out turbos. MPGs didn’t improve much in the 90s – 2000s but they cranked out big power while keeping clean emissions.
Currently, heat issues has been addressed and lag continues to diminish. Turbo is the immediate future. It is a perhaps a stop gap to achieve conflicting goals of power and efficiency but I don’t feel it is innovative.
Laser ignition and infinitely variable timing and valve lift systems, which can operate reliably at high rpms, will be warmly welcomed when they arrive. Six plus years ago, I spoke with a research scientist at the DOE who was working on laser igniters for automotive applications. It is slow coming but I believe it will happen. This industry is researching new innovative concepts we haven’t imagined. Will turbo be prevalent 20 years from now? Maybe, or maybe not.
