VE doesn't correlate linearly or anywhere near what you think and especially when a setup is untuned. There are too many variables involved. With a large injector, at low pulse widths even .100ms (100uS) is a long time and can significantly alter a/f ratio. Fuel injector size input plus delay is already significant error margin. Tuning a setup can include frequency response of pulse driver circuit, literally the tuning of transistors to desire optimal injector on-time for extremely low pulse width targets such as when trying to idle 650rpm with 210lb/hr injectors on pump 93 octane gasoline. To be precise, it isn't an issue of VE target on a fuel map, most of the on-time is coming from calculated delay periods based on manifold pressure, temperature, fluid density, and voltage dependency, and the injector has a preferred duty cycle range. To put it another way, the latency delay feature may result with a fueling over-correction much more frequently at low rpms in much larger magnitude than is possible to make using fuel adjustments from the VE table. The VE table contributes very little on-time to a large injector at idle so even large VE values such as 100% compared with 50% VE values, both result with similar amounts of fuel flow and power is very similar because overall not much more fuel is going into the power plant because the rpm is only 600rpm and the engine is only lightly loaded. It couldn't even melt itself if we removed all timing advance and ran it lean or rich, it would never overheat because power throughput is so low no matter how crazy we get with the VE map at such a low rpm. Therefore when you see large changes to fueling you don't look at the VE Table really. You look at offsets, injector size calcs, minimums and maximums, multipliers, and check your airflow calculations to see how much power the ECU thinks its making and compare that with what it used to make at idle or what the stock one used. Duty cycle injector flow rate back calculations using manifold pressure and injector duty cycle reports are still partially skewed by fuel density due to fuel temp. Realistically calculating bsfc and controlling power throughput to achieve max economy for each situation is challenging unless you are measuring all these tiny extra data points and accounting for energy, energetics of fluids, speaking of whichOriginally Posted by cmitchell17
We lose energy by attaching a turbo or long intake tubes, but its not that bad for turbos with long tubes.if anything, would be rich with the stock VVE table because of the extra restriction of the turbo compressor and intercooler and piping causing less actual VE.
Adding tube adds friction. Thus intake tubes on naturally aspirated engines should be pretty short to minimize friction losses. Engineers often calculate Reynolds numbers and find pipe roughness and fluid velocity over some length of tube to determine energetics of fluids, how much power it will take to pump so much fluid how far in what time can all be calculated easily using online engineering toolbox calc applets powered by coffee.
Adding tube length adds friction. Intercoolers reduce kinetic energy, they throw away energy, the power will be reduced. Intercoolers eat power!
So simply put, the engine will use more fuel. Economy is worse with longer pipes, more friction, same power requirement to the tire, more energy input needed.
Could you raise the temperature of the power plant, improving efficiency and offsetting some of the friction losses? Yes
Could you attach a turbo, reducing efficiency and offsetting some of the friction losses? Also yes.
The turbo will reduce efficiency of the engine but offset kinetic energy losses of long pipes because the compressor impart kinetic energy to air molecules entering the plumbing using energy it took from exhaust energy. That is because kinetic energy losses are enormous for a typical front mount intercooler setup, whereas the efficiency drop due to high exhaust gas pressure is minimal under the correct circumstances such as properly timed injectors and valve events. Making cam selection and injector size both major factors in the fuel economy department, much larger than the engine's running a/f ratio.
The turbine extracts energy from exhaust gas which drives a compressor which lends kinetic energy to incoming air molecules propelling them through the intercooler plumbing thus making up for the friction losses of having long intercooler pipes and almost restoring fuel economy despite lots of pipes with friction.
However. There are still many losses to weigh in on the exhaust side. Higher exhaust gas pressure protects the rod cap through TDC and turbo engines are far more RPM friendly due to the exhaust cushion provided by exh gas pressure. However this pumping loss during economy situations impedes flow and disorganizes the flow, when compared with an individual header system where each exhaust pulse is preserved and the energy or flow work of the exhaust gas pulse flowing in a tube causes a cylinder evacuation/scavenging low pressure affect which provides much energy to move exhaust gas out and clear the cylinder, possibly draw in fresh air and fuel and even some of that could then leave the exhaust. Thus we see that even economizing our cylinder clearing-ness by provided free energy for removal some fuel could be wasted ultimately resulting with poor-er economy. Thus valve timing and fuel injector end-of-time are probably central to achieving max economy and no amount of VE table tuning is going to do that for you, calculate exh valve closure and figure out how long the pulse is at whatever rpm and then time the pulse to fit within that window when possible during a cruise to conserve fuel. The VE table is practically linear and perhaps the most uninteresting and un-tunable table in the entire ECU. It is those injector delay period voltage tables and timing cylinder reference end of injection tables where the real tuning happens. You could interpolate a random VE table for a random engine and then tune every other variable around its generic shape and achieve the same end result as if you pain stakingly adjusted every steady state VE table point. Either way you will still have to go around blending and tweaking every single setting no matter how good the VE table is because it interacts with everything, so what difference does it make if the PE value in the box is 1.21 or 1.35 you would still need to empirically test and then adjust the value at least a single time and the amount of time to do so isn't less no matter how good the VE table is making all of the work pointless since you still wind up at the same final a/f value at wide open throttle.
The other thing is, with a really good flowing turbo at cruise or idle conditions, there is alot of kinetic energy being thrown at the intake manifold and throttle body. The IACV will actually have to close some, and the throttle body tighter, and the vacuum will increase, which means location of idle MAP/VE position will also change. More vacuum and more fuel requirements at the same time. Lets finish with the fuel requirement though, finish the thought. Higher intake vacuum means what, it means more energy required during intake stroke, the piston surface area fights the increased vacuum and more energy is lost there. Thus turbochargers investing kinetic energy at the intake manifold will reduce economy in the big picture because even if we offset all of the friction losses from all of the plumbing (which I believe is almost completely possible) the piston still fights increased vacuum as a result resulting with a net loss in energy. Because- we know exhaust gas pressure and pumping loss during exhaust stroke will be higher or at least the same. It can't get better, higher exhaust gas pressure isn't ever going to be better. Power stroke we assume the same amount of energy requirement, and compression is a pure energy loss no matter how you slice it. Economy will be reduced: loss, loss, loss, loss all 4 strokes is a loss or equal to before when adding a turbo or any length of tube anywhere. More fuel requirement given the same vehicle and conditions.
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