Simple Tachometer Design for HCCI engines

[dwightlooi]

If we take a step back and reconsider the good old Tachometer, one has to ponder... does all those markings and graduations matter?

I mean do you really care if to know if the engine is making 4153 rpm? If you do, most tachs are not marked that precisely anyway. If all that is important is knowing roughly where you are in the rev range then do all those graduations serve only to clutter up and confuse?

In the course of daily driving -- even spirited driving -- what really matters really are the basics. Is the engine idling at the intended RPMs? Am I about to bump the rev limiter? Where I am on the rev range? Am I crusing at an economical engine speed?

I propose a very simple tachometer design for future HCCI equipped vehicles. The gauge has no markings except:-

• A "0" denoting engine is off
  
• A single notch indicating the warm idle
  
• A blue band from 1500 to 3000 rpm indicating the RPM range over which HCCI may operate
  
• A red "7" and band indicating the redline
  

If the driver really whats to call up the exact rpm for diagnostic purposes (which is rare) he can call it up amongst one of the modes on the text display which also serves as the odometer, trip computer, mpg readout, etc.

All the other nonsense are simply eliminated. I think it's a starkly clean look! What do you think?

Edited March 21, 2011 by dwightlooi

[Drew Dowdell]

I like...

The old Saab 9000 had a tach with an orange range labeled "Efficiency" or something like that. The premise being that if you kept the tach in that range while accelerating, you were getting the best fuel economy you were going to get.

[67impss]

On HCCI engines operating in the blue band is that the torque band if not I'd like to see that indicated & a hash mark @ max HP

[Robert Hall]

What's HCCI? Not familiar with that FLA. I can't recall the last time I actually looked at my tach.

[Drew Dowdell]

Homogeneous Charge Compression Ignition.

It's a gasoline engine that works like a diesel (not using spark) under certain conditions. Direct injection is required for it to operate correctly. Provides very high fuel economy numbers, but so far has issues with NHV when "shifting" modes.

[loki]

useful for the "everyperson". but i like alittle more info than what you have on it.

[balthazar]

^ Agreed.

[SAmadei]

My only stick car actually has no tach. You shift by ear or by the little upshift ticks on the speedo. Thats minimalistic.

[Z-06]

Actually the use of a tach is beist for a heel-toe downshifting a stick-shift that is about it. Based on the engine speed of a higher gear one can estimate the speed for the lower gear he/she wants to shift into.

As far as upshift is concerned, I agree with SAM that engine noise can do the job.

Good idea Dwight.

[Intrepidation]

My only stick car actually has no tach. You shift by ear or by the little upshift ticks on the speedo. Thats minimalistic.

Same, that's the one car a tach would actually be useful, yet the Prizm doesn't have it.

[dwightlooi]

Homogeneous Charge Compression Ignition.

It's a gasoline engine that works like a diesel (not using spark) under certain conditions. Direct injection is required for it to operate correctly. Provides very high fuel economy numbers, but so far has issues with NHV when "shifting" modes.

Oh there are all kinds of problems; NVH is just one of them. Actually, the NVH is not that bad. Reports indicate a slight "ringing" afternote to the engine sound; not particularly obtrusive. The biggest problems are three fold...

To have HCCI function, you need to (1)know the precise pressures within the cylinders in real time, (2)be able to control it and (3)be able to precisely meter and time the introduction of fuel. If you can't do any of these you can't have HCCI. The last problem is generally a non-issue with the advent of direct injection. The first requires a reliable and long lived pressure sensor -- that is being worked out. This leaves the ability to control pressures in real-time. This is the hard part.

There is basically only one way to control cylinder pressures -- by varying compression ratio based on temperatures, speed and load. One way you can do this by raising or lowering the deck height relative to the crankshaft, but nobody does that (actually Saab tried it once, but not very successfully). The most practical means of controlling effective compression ratio in real-time is to have a continuously variable intake valve opening duration. By closing the intake valve late past BDC, the engine ends up kicking part of the intake charge back out of the cylinders instead of compressing it. This reduces effective displacement (amount of air actually remaining in the engine) and effective compression (the less air remains, the less is compressed against the same combustion chamber size at TDC, the lower the compression ratio). To continuously vary intake duration, most solutions use two separate camlobes and an additive rocker to operate an intake valve. The valve only opens when both lobes are not on the base circle. Usually, the intake cam is actually a cam-in-cam setup so the timing of one lobe can change relative to the other to alter the intake duration.

Even with all these elaborate controls, GM has been struggling to get HCCI to operate above 60mph -- actually it's more like above the top gear engine rpms at 60 mph (about 2500 rpm). Go to a taller set of gears and there isn't enough power in HCCI mode at freeway cruising speeds. Lower gear ratio and the HCCI crusing speed falls below 60mph. I think that once they can get HCCI to work between about 1500~3000 rpm or even 2000~3000 rpm, it'll be ready for prime time.

In all likelihood, HCCI engines will be Triple-cam engines. Or, more precisely speaking, a DOHC style engine where the intake cam is actually a concentric cam-in-cam setup.

[regfootball]

i like to see what rpm i am idling at, what rpm i am cruising at in a particular gear. especially when going 70+ mph on trips.

with something like a CVT too a tach can be a nice story of how the engine rpm is not rising in complete correspondence with the vehicle speed.