Atkinsoning motorbike engines

[nemesis]

There are lots of variables that you can change and retarding the intake cam does sound like it would decrease dynamic compression/horsepower/torque which should decrease pumping losses in theory and less power would requier less fuel, but altered ignition maping along with fuel mapping with vvt timing should make this combo work pretty good. Vvt would retard intake cam while you're cruising and when you need more power it would advance the intake cam to gain more power. It would be tricky to get igntion/vvt/fuel timing to work all at the same time on top of other variables like map sensor, air intake temp, engine coolant temp, oxygen sensor, bap sensor, tps sensor.

[renault_megane_dci]

Originally Posted by nemesis

There are lots of variables that you can change and retarding the intake cam does sound like it would decrease dynamic compression/horsepower/torque which should decrease pumping losses in theory and less power would requier less fuel, but altered ignition maping along with fuel mapping with vvt timing should make this combo work pretty good. Vvt would retard intake cam while you're cruising and when you need more power it would advance the intake cam to gain more power. It would be tricky to get igntion/vvt/fuel timing to work all at the same time on top of other variables like map sensor, air intake temp, engine coolant temp, oxygen sensor, bap sensor, tps sensor.

VVT as it is done on car engines is very challenging to retrofit on a bike engine for a DIYer.

We are not dealing with car engines where the high torque at low RPM is mandatory to make the vehicle move, we are detuning overly powerful engines to make them reach a more acceptable FE to capacity ratio.

If a 500 bike was doing 195 km/h and return 45 mpg, I don't mind it is modified to hit only 130 km/h but return 80 mpg.

But I don't know how to extrapolate either MPG improvement or power loss induced by Atkinsoning of those engines.

[nemesis]

I was working on vvt set up on my bike, but it was going to advance in the low/mid range to gain power and retard as the rpms went up, so the engine would just build power all the way till rev limiter and still have nice flat torque curve.

[renault_megane_dci]

Originally Posted by nemesis

I was working on vvt set up on my bike, but it was going to advance in the low/mid range to gain power and retard as the rpms went up, so the engine would just build power all the way till rev limiter and still have nice flat torque curve.

How far have you been ?

To my knowledge, most of the VVT set up are pressure oil activated (there is an oil cavity inside the pulley of the camshaft that is filled on demand and retard or advance it)

Advancing the cam requires deeper valve cut outs, it $$$tarts to add up pretty fast.

[nemesis]

Originally Posted by renault_megane_dci

How far have you been ?

To my knowledge, most of the VVT set up are pressure oil activated (there is an oil cavity inside the pulley of the camshaft that is filled on demand and retard or advance it)

Advancing the cam requires deeper valve cut outs, it $$$tarts to add up pretty fast.

Yes I've looked up most set ups and most of them operate by oil pressure, I was making a set up that worked of off centrifugal forces, it would start off at around 105 deg intake cam centerline ( I can't go any lower or the valve would hit the piston), and around 10k rpm would start retarding to around 111 deg cam centerline.

[renault_megane_dci]

sounds even less DIYer friendly ...

[nemesis]

I will try to get it done once I am done with more important things on the to do list.

[renault_megane_dci]

I searched the web today trying to find out % of power loss and MPG increase for Atkinson engine but have not been lucky.

Since most cars using the Atkinson cycle have cam phasers, the switch from Otto'ish to Atkinson'ish depending on power requirement.
So their overall performance and MPG is not gonna be properly reflective of a fixed Atkinson as I intend to build.

Also, I have been looking at SV's (I like Suzukis) since they are far more sexy than GPZ's but had to rule them out since they would need two cam regrind instead of the cam swap I intend to do and they have a 150 cm3 penalty in the FE race ...
Honda CB 500 are out of the question too with no lower power derivatives.

Ho well, I'd better get used to drive a Kawi ...

[mechman600]

Originally Posted by renault_megane_dci

Since most cars using the Atkinson cycle have cam phasers, the switch from Otto'ish to Atkinson'ish depending on power requirement.

A true Atkinson engine does not switch to the "Otto'ish" cycle when more power is required. Doing so would create holes in pistons due to the higher than Otto compression ratio. They do vary intake valve timing at various speeds and loads, but only a small amount.

Atkinson engines (in the modern sense) require:
1. Increased static compression ratio
2. Deliberate reduced volumetric efficiency via delayed intake valve closing

If one of these is missing, it is not an Atkinson cycle.
1. Increasing static compression ratio without delaying intake valve closing will make things go bang (in a bad way).
2. Delaying intake valve closing without increasing static compression ratio will decrease pumping losses slightly and increase efficiency slightly, but not nearly as much as with a true Atkinson cycle.

In order to make your motorcycle an Atkinson engine, you must raise your compression ratio. You must also ensure that you have reduced the volumetric efficiency enough that dynamic compression will not even get near static compression ratio and ruin your engine.

IMHO, creating a VVT system is pointless...an unnecessary complication. For your purposes:
1. Compression: plane the head [like crazy] and install a thinner head gasket (if available). Measure combustion chamber volume before and after. Static compression ratio = (swept volume + combustion chamber volume) / swept volume.
2. Intake valve timing: weld and grind the intake camshaft lobes for delayed valve closing. Install a degree wheel on the crankshaft and dial indicator on each intake valve as you weld and cut and grind each lobe.
3. Write everything down.

[renault_megane_dci]

Prius is only 13:1 static.
This gives 8.2:1 dynamic with the advanced cam phasing but only 6:1 with the late cam phasing.

On the GPZ (pretty much set on this now) I was aiming for restoring the standard dynamic CR wich is 7.261.