Cam timeing effects on fuel econnomy

Hey

So you have a course on power and emissions however how do you affect fuel econnomy regardless of power and emissions?

Or is emissions enherently corralated with econnomy?

I know that toyota say that vvti is basicly used as a form of "internal EGR" when cruiseing but i dont see this covered

Any pointers on this? Maybe a general fuel econnomy course would be intresting as well to also get the most possible out of you engine also in daily use

There are two different drivers for the efficiency for maximum torque/power compared to maximum fuel economy.

For the first, we want as much mean cylinder pressure as we can from the amount of oxygen in the air, and this means running more fuel than stoich', for various reasons.

For the latter, we are looking at using the fuel as efficiently as we can to maximise the power per unit of fuel. The thing that most overlook is it isn't actually the fuel that gives the power, but the use of the fuel to heat the gases (including the combustion ones) in the cylinder to produce pressure that we use to drive the piston down. Because of several factors, that means a leaner than stoich' mix - often called "lean best power", "lean cruise", or similar terms. Different engines respond differently to where this lambda lies, but 1.10 to 1.15 is the expected range. More on this a little later.

Depends on which emissions you have in mind - carbon dioxide is going to be in direct relationship to mass of fuel used, and will vary with the C-H, and possibly O, molecular balance. Carbon monoxide is a result of rich mixtures where there wasn't enough oxygen and/or poor combustion. Hydrocarbons are usually due to misfires where the fuel isn't burned, but can also be affected by poor combustion. Oxides of Nitrogen are a result of high combustion temperatures with lean mixtures, where the atmospheric nitrogen 'burns' - this can be a problem with some diesels, and is why petrol/gasoline engines are often run at stoich' from the factory, as it burns a little cooler and doesn't leave unused oxygen for the Nitrogen to attach to.

I would assume Toyota are adjusting the exhaust closing and inlet opening to trap some of the exhaust gas in the cylinder, this can have advantages for emissions, such as the residual exhaust gas reducing cylinder temperatures a little and so reducing things like the NOx mentioned above.

I very much agree - there are a LOT of ignorant and/or stupid people who run excessively rich everywhere - not only can this make a BIG difference to fuel economy it also contributes to plug fouling, bore wash and resulting ring and bore wear, oil dilution, etc.

Another aspect that's often ignored is the spark plug timing - people will set it for the maximum boost/power and leave it there, when having it advance under light load can make a big difference with fuel economy, part throttle response, spark plug life, etc.

I can't really comment about the valve timing, but following the usual, fixed fine tuning advancing the intake will be expected to increase low rpm cylinder filling for torque, and probably the same for the exhaust. Depending on your resources, and vehicle, it may be something to play with in your own time. For example, many modern vehicles have an instantaneous fuel consumption read-out and cruise control - you may be able to set the cruise control at "x" speed, and advance/retard the cam'timing(s) to see what that does to the fuel useage? best if someone else drives if you're making adjustments and checking the usage. Similarly, you can adjust the timing, preferably with knock sensors and/or headset. Remember, each will affect the other.

Hi Gord so advancing the timing will hurt fuel economy?

"Another aspect that's often ignored is the spark plug timing - people will set it for the maximum boost/power and leave it there, when having it advance under light load can make a big difference with fuel economy, part throttle response, spark plug life, etc."

Advancing helps fuel economy. Just as a more compressed (denser) mixture - as found under full throttle/boost - has a faster flame front (burns faster) and needs to have timing pulled, a less compressed (less dense) mixture - as found at cruise and light throttle - has a slower burning flame front and needs the ignition advanced to have the cylinder pressure develop fast enough for best mean cylinder pressure balance - best bang for the buck, so to speak.

It is possible, in some cases, to over-advance beyond MBT, but usually keeping it just short of detonation works best.

Best way is usually on a dyno, but you can get quite close on-road, especially if you have a vehicle with a fuel read-out display.

Something I failed to recall for the previous post - an old school method was simply using a vacuum gauge hooked up to the manifold and trying different vacuum diaphram assemblies - with EFI you can do the same thing, but BETTER!

When you have the highest vacuum that's because the engine needs less air past the butterfly and entering the engine for holding the speed because it's making the best use of what's being burned in the cylinder..

Nice thanks Gord! That's what I was thinking. My first turn will be done on a 1995 VW 2.5L 5Cyl with Digifaint ECU.

No cam control on this engine but it was designed to make the most torque by 2200RPM. When I tune I'll try for the dyno if possible.

My goal was best power at WOT and best fuel economy at cruise around 100-110KPH or around 60 mph here in the US. I will read all the info first on the forums before trying and then post.

Awesome info from Gord!

Adjusting fuel targets will have a very predictable and sizable impact on fuel economy. Once you get the fuel side dialed in, there may be a tiny bit left in ignition and cam phasing, but keep in mind unless the tune is way off in the weeds, small changes will have such a small impact, if any, on fuel economy, that trustworthy measurement can be a challenge.

Steady state testing on a sensitive, low inertia dyno will really help you observe small changes in torque output for a given fuel input.

If you attempt to analyze results achieved on the road, keep in mind where the fuel economy readout comes from, fuel delivered over distance. Unless you're maintaining perfectly constant speed, on a perfectly consistent stretch of road, with no surface or grade changes, no changes in wind, etc., that reading will be far more impacted by varying test conditions, than by reasonable changes to cam or ignition timing.

If you've had dyno time since your last post, I hope it's gone well.

Have a great day,

Hi Mike thanks. The camper is a project and going threw the fuel tank now then need to sort out other items. I hope it will be on the road in 2024 and then I'll try the dyno if I can. If not, ill use a wide band and just target a 15.5-16 AFR at cruise.

George,

I'd monitor cat converter temperature as running that lean may potentially turn it to moon rock and they're likely expensive to replace.

Hey guys!

Alot of very nice info here

But some new questions pop into mind

1. More vaccume is a good indicator for fuel econnomy - gord

- i have done a supra recently with stock sequential twinturbos that i can control with the standalone, and running 2 turbos will mean you get more vacume anf need to give it some more gas to keep the speed, and you will notice a very sharp "lift" when turning the second turbo off ( cos the first turbo will come on boost at highway speeds) and you will need to get off the gas to maintain speed however the vaccume will also stay higher that with 2 running

What is better for fuel econnomy in this case and why? Cos on one side you are restricting the exhaust ports by running one turbo, however you are getting more air in the cylinders potentialy makeing a more efficient charge? The vaccume in the manifold if ofc less but since you can lower the tps snd keep the speed... alot of things and i cant see a clear winner, my heart tells me 1 turbo useing last exhaust energy to pump air if the more fuel efficient solution but on the other hand more are just means adding more fuel haha so i dont know

And since i can control it, might as well set the turbos to run whatever is cheaper when cruiseing

2. When i have a stanalone on the car, its it not possible to simply look at the injector duty/fuel load at constant speed on the highway and simply bring that down as low as possible that still maintains speed?

3. Tempratures higher at leaner mixtures

My understanding is that a perfectly stoich combustion is always the max possible temp, but i guess at stoich mixture you dont nessesaryli have a stoich combustion do therefore adding more air makeing it lean will make more o2 avaliable to surely burn all the fuel

But i would then also say dont all fuel then burn in the chamber makring the exhaust temps lower by proxy ( so no danger to catalytic converters)

Simon,

I can't speak specifically to sequential turbo control related to fuel economy on that particular application, but in general principle if you have a stretch of relatively straight relatively flat road without traffic, perhaps 1 mile long, and you make a pass in both directions and log each attempt, maintain a speed for the whole distance you log, and average fuel usage over the tests in both directions, you can ballpark average fuel economy for that configuration, then make a change, test again and compare. I use a 1 mile range as tenths of a mile are marked on some highways in my area, but if you have a 1km stretch for example, that could certainly work as well.

In terms of temperature, I'd stick to stoich for a combination of cat converter health, efficiency, emissions. Cats are really designed around efficient operation at stoich, and constructed to withstand the heat at stoich, so additional heat at leaner mixtures can cause them to fail.