While searching for information about how is engine load calculated I came across this question which contains an equation for the ECU load calculations.

As I understand it under fixed atmospheric conditions engine load depends only on current airflow and RPM (which determines peak airflow at wide open throttle), as other members of the equation can be considered to be constant.

However I don't really understand how can different load values be measured on a naturally aspirated engine when quickly fully opening the throttle in neutral and climbing a hill at WOT in top gear. As far as I know 100% load value can't be reached in neutral, why? Fully open throttle should allow maximum airflow for the RPM in neutral and hillclimb.

EDIT: After testing on my own car, 100% load indeed CAN be reached, but only for a very brief moment, it drops before max revs are reached. Not quite sure what causes this behavior.

4 Answers 4


Air flow is purely based on throttle position and engine revs on a normally aspirated engine, adding a charger (turbo etc) adds complication. With the throttle fully open, every inlet stroke is going to take the maximum amount of air (and therefore fuel) into the cylinder. This air intake can then be multiplied by the engine revs. If the throttle is not fully open, then quite clearly you are restricting the air flow into the engine and so each inlet stroke will not be at full air/fuel capacity.

You are correct in that if you put your foot flat to the floor in neutral, you will allow maximum air in and the revs will quickly build up to the rev limiter. All the power produced by the engine will be going in to accelerating the engine and flywheel etc at it's maximum rate. The acceleration of the mass of the engine is in effect the load on the engine.

  • So basically flooring it in neutral will cause 100% load on NA engines? What about turbocharged engines then? As far as I know the boost pressure is determined by engine load and rpm. But turbocharged engines never allow high turbo pressure in neutral, why is that if it is possible to reach highest load? Nov 16, 2015 at 12:21
  • When you say load, presumably you are meaning that the engine is putting out its maximum power possible at the current engine revs, is that what you mean? I think there is a little confusion with the terms here, I believe you are wanting to determine the load on the engine from the current power output of the engine. If the turbo adds more air and hence more fuel is added, then the engine is able to produce more power and hence drive a larger load.
    – HandyHowie
    Nov 16, 2015 at 13:07
  • Load is a term frequently used in engineering to mean the force exerted on a surface or body. Force is mass multiplied by acceleration. So to increase the rate of accelerate a vehicle your engine has to produce a larger force. See -diffen.com/difference/Force_vs_Power
    – HandyHowie
    Nov 16, 2015 at 13:18
  • No, let's go back. I am talking about load as defined by SAE J1979 / ISO 15031-5 in the question I quoted and is available via OBD, are there different "loads"? According to the equation load depends on airflow and rpm. Is this not the same load that is taken into account when ECU is calculating the peak allowed boost pressure? Nov 16, 2015 at 14:38
  • The last question you asked didn't make sense - "why is that if it is possible to reach highest load?" If the turbo was able to spin up quickly enough and was allowed to produce a large boost in neutral, the engine revs could spin up that quickly that it could run out of control.
    – HandyHowie
    Nov 16, 2015 at 15:38

Doesn't it suck up only as much as engine speed and throttle position allows?

No, throttle position and engine load determine the quantity of air consumed.

It can be quite hard to understand at first how a naturally-aspirated engine can ingest different quantities of air at the same RPM.

Here's what the Engine Management Fundamentals chapter of the Bosch Fuel Injection & Engine Management book has to say on the topic:

Fuel delivery requirements depend more than anything else on how much work you are asking the engine to do - or how much of a "load" you are placing on it. To accelerate, you step down harder on the accelerator. This opens the throttle valve, increasing manifold pressure. The greater pressure difference between the manifold and the cylinders increases intake air flow, and therefore fuel flow, to increase power and accelerate the car.

Driving down a level road, you can cruise along comfortably and maintain a desired speed with a relatively small throttle opening. When you come to a hill, it is necessary to pressure farther down on the accelerator to maintain the same speed, even though engine rpm is unchanged. The hill has demanded more work from the engine - created a higher load - and the engine has demanded more air and fuel to match that load.

Regardless of engine speed, the air flow and fuel delivery demands of the engine depend on the load being placed upon it. That load, and the resulting throttle opening, directly affect manifold pressure. Manifold pressure in turn affects air flow and thus fuel requirements.

The above-quoted should be enough to answer your question, but here are my original musings on the topic:

  • The internal combustion engine is a volumetric device

    In other words, it operates by taking in a certain volume of air-fuel mixture during the intake stroke. This is important to keep in mind because...

  • Volumetric efficiency impacts how much air and fuel is actually taken in

    So a 2.0 L 4-stroke engine could be running at 2000 RPM, and one might expect that the engine should consume 2.0 * 2000 / 2 = 2000 L of air-fuel mixture per minute, when in reality it consumes something closer to 1700 L. The reason for this is volumetric efficiency, which is the ratio of what is actually consumed to the theoretical consumption based on engine size and speed alone.

  • Volumetric efficiency is affected by load

    Let's build on the car in neutral vs uphill example by adding a third scenario where the car is running on level ground. The torque required by the engine to maintain a certain speed will vary depending on the external loads on the vehicle as per the ASCII diagram:

    LOW                                                    HIGH
    --------------------------  TORQUE ------------------------>
    Neutral                     Level Ground         Uphill
    [2000 RPM]                  [2000 RPM]           [2000 RPM]
    Auxiliaries                 Auxiliaries          Auxiliaries
                              + Aero Drag          + Aero Drag
                              + Drivetrain         + Drivetrain
                                                   + Car Weight

    Different torque ("load") demands will result in the engine altering the volumetric efficiency to adjust the air-fuel mixture accordingly.

    So the load on the engine governs the volume of air/fuel that is ingested. This is also why it is possible to determine the absolute load on the engine if one knows the volumetric air flow and engine speed.

  • I would modify your first point about throttle position for clarity: the throttle position is more of an indication of your desire for a change in the engine speed. Closed == return to idle speed. At the current equilibrium point, no change desired. WOT = maximum possible acceleration. That might help with your later explanation of why the engine could be gulping different amounts of air for a given RPM. It's really a first derivative control rather than direct.
    – Bob Cross
    Nov 16, 2015 at 20:23
  • You lost me in this part: "Different torque demands will result in the engine altering the volumetric efficiency to adjust the air-fuel mixture accordingly". How exactly does the engine alter the volumetric efficiency? If the RPM is constant then the only way to fill the cylinders more is to open the throttle (atmospheric engine). But that can be done both in uphill and neutral. Nov 17, 2015 at 10:40
  • 1
    @IhavenoideawhatI'mdoing : Yes, you'd need to add more throttle to maintain the same RPM. Note that the throttle position required to maintain a certain RPM will depend on the load.
    – Zaid
    Nov 17, 2015 at 10:54
  • So how does that explain if maximum load can be reached in neutral or not? I understand that maintaining a certain RPM in neutral and uphill requires different throttle positions and results in different airflow and load. However if you open the throttle in neutral then by the time you hit the rev limiter you have maximum airflow to the engine, highest volumetric efficiency and therefore - 100% load? Nov 17, 2015 at 11:01
  • 1
    @IhavenoideawhatI'mdoing : engine load at max RPM with the gear in neutral will not be 100%. The fact that it hits the rev limiter is either because the throttle is not longer at WOT, or that something else like valve timing has changed to alter the amount of air & fuel demanded by the engine. I assume your reasoning is that air flow is a function of throttle position and engine speed, when it is more a function of throttle position and engine load. I'll update my answer to quote a more authoritative resource than yours truly :)
    – Zaid
    Nov 17, 2015 at 11:41

Engine load is determined by a ratio between current air flow and the maximum air flow at the same RPM.

The engine computer has a look up table of maximum air flow as a function of RPM for WOT values. This table is generated by the manufacturer using an engine dynamometer. To generate the table the engine RPM is held constant (with the dynamometer) and the throttle is held wide open to get the value. This is repeated for all RPM values at standard pressure and temperature.

The equation you quote gives you a fraction of maximum load by dividing current air flow by maximum air flow (as picked from the look up table) and compensated for current barometric pressure and current temperature. This is very fundamental for naturally aspirated engines because at any RPM there is only a single maximum air flow value.

When you talk about super charged or turbo charged engines the only difference is that it has a different look up table of maximum air flow at WOT. This look up table that is just like for the naturally aspirated engines. The table is also generated in a similar manner where the engine RPM is held constant at a WOP condition but then the boost is varied from naturally aspirated to maximum boost. This generates a multi dimensional look up table where every RPM has multiple maximum air flow rates depending on boost.

From here the boost management depends on the specific manufacturer. Some may limit boost in neutral some may not. But the calculation depends on current air rate and the value from the look up table that is found using current RPM and current boost.

  • So again, 100% load can be achieved in neutral. In that case, what do you think limits the boost pressure in neutral, if it's clearly not (only) rpm and load? Wheel speed sensor? Some kind of gear sensor? Nov 16, 2015 at 16:55
  • @IhavenoideawhatI'mdoing What your asking is beyond theory and into application. Every make, model and year will manage it's boost differently per their design. Unless you want to give us a make, model and year of the car your interested in we can only guess at what you want. There is no universal model here.
    – vini_i
    Nov 16, 2015 at 17:59
  • @IhavenoideawhatI'mdoing I disagree with the assertion that 100% load can be achieved in neutral. If you have a scan tool that measures air flow rate you should be able to see that the value at max RPM in neutral vs max RPM with the car running are quite different
    – Zaid
    Nov 16, 2015 at 19:31
  • @Zaid What would you expect to see at full throttle in neutral as the revs approached max RPM? I am not saying you are wrong, just wanting to understand it.
    – HandyHowie
    Nov 16, 2015 at 21:28

100% engine load is the situation where the throttle flaps are wide open and fuel system is providing as much fuel as possible and the engine is at peak torque but the revs are not rising. This type of scenario can be seen in the real world when a vehicle with a heave trailer attached is climbing a steep hill, in a low gear and with foot flat to the floor, is not getting any faster.

This scenario can be simulated on a rolling road. It can not be achieved in neutral because there is insufficient opposing force on the flywheel to counter the acceleration of the engine. What will happen is that RPM will stop climbing when the rev limiter cuts in or the bob weights on the distributor can not advance the ignition any further. This is not the same as opposing forces slowing the rate of RPM rise to zero.

  • I'm intrigued by the "but the revs are not rising" part. How is the acceleration of the engine taken into account? That equation I pointed to only seems to care about airflow, but after trying to check ECU calculated load with an OBD scanner this makes sense - after flooring it in neutral load shoots to 100% and drops instantly when rpms start rising. Dec 3, 2015 at 9:55
  • The way a rolling road works is it provides opposition to the engine and the measure of that opposition allows you to calculate how much force is coming from the engine and how much resistance would overcome that force. Once you know what that resistance is, you have a figure for the ultimate load that could be applied to the engine. I suspect your OBD scanner is giving you a false figure. Bear in mind that it's only estimating load based on air flow and fuel demand. If this were being applied and the revs of the engine weren't increasing, you'd have an approximation of current load. Dec 3, 2015 at 13:36
  • I understand what load is, this question is purely about the figure that the ECU calculates. It's not giving me a false figure, it's giving me the figure that the ECU gets and relies on (which usually does not match actual external load). Dec 4, 2015 at 7:16
  • The estimation is what concerns me - how? The question I quoted seems to answer this - it basically depends purely on airflow, but it doesn't explain the behavior I see, as full throttle would always get me 100% load figure, even in neutral. But what happens is it jumps to 100% for a very brief moment and reduces, as if it depends on engine acceleration, as you said. Dec 4, 2015 at 7:16

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