Engine throttle/resistance question
Engine throttle/resistance question
(OP)
I have been given the job of creating a dynamic car model in software at the company I work for. Being a computer engineer, it has been quite an effort for me to get to know cars and the dynamics thereof. As guidelines, I have the following three books to help me:
Fundamentals of Vehicle Dynamics - Gillespie
Motor Vehicle Dynamics - Genta
Race Car Vehicle Dynamics - Milliken & Milliken
These books have proven to be invaluable, but there are still some questions that aren't answered which I just can't figure out for myself.
First question: how does the throttle affect the torque delivered by the engine at a specific engine speed? I assume the torque table gives the max torque at a certain engine speed, thus if the throttle is at maximum, that is the torque delivered. I further assume the torque is scaled linearly with the throttle, thus if the pedal is pushed only half way, you only get 50% torque at the specified rpm value, etc. If this is incorrect, please inform me.
My second question is this: if the car is idling, the throttle is probably at some minimum level, just enough to keep the engine running. I assume this level probably can't be computed mathematically, and each car manufacturer just approximates this value by trial and error. Also, in the above mentioned books several equations are given to determine the engine torque, the torque at the clutch, the torque to the drive shaft, the torque to the axles, and the torque to the wheels, which are all pretty straight forward. The only problem I have is to get my engine to idle correctly. I know that is my car is in neutral and the engine is idling, that there is still some resultant torque delivered from the engine, but why isn't the engine speed increasing all the time? According to the equations I have, this is what should happen. So I think one of the following is possible:
1. I shouldn't scale torque linearly with throttle
2. There is some engine resistance factor that is not shown in the equations, that works against the torque delivered by the engine to keep the engine rotating at a constant speed.
It seems as if all this should be pretty straight forward, since the books don't really bother all that much with the engine and drive train, but instead focuses on the tyres and suspension dynamics.
A pointer in any direction at all would be greatly appreciated!
Fundamentals of Vehicle Dynamics - Gillespie
Motor Vehicle Dynamics - Genta
Race Car Vehicle Dynamics - Milliken & Milliken
These books have proven to be invaluable, but there are still some questions that aren't answered which I just can't figure out for myself.
First question: how does the throttle affect the torque delivered by the engine at a specific engine speed? I assume the torque table gives the max torque at a certain engine speed, thus if the throttle is at maximum, that is the torque delivered. I further assume the torque is scaled linearly with the throttle, thus if the pedal is pushed only half way, you only get 50% torque at the specified rpm value, etc. If this is incorrect, please inform me.
My second question is this: if the car is idling, the throttle is probably at some minimum level, just enough to keep the engine running. I assume this level probably can't be computed mathematically, and each car manufacturer just approximates this value by trial and error. Also, in the above mentioned books several equations are given to determine the engine torque, the torque at the clutch, the torque to the drive shaft, the torque to the axles, and the torque to the wheels, which are all pretty straight forward. The only problem I have is to get my engine to idle correctly. I know that is my car is in neutral and the engine is idling, that there is still some resultant torque delivered from the engine, but why isn't the engine speed increasing all the time? According to the equations I have, this is what should happen. So I think one of the following is possible:
1. I shouldn't scale torque linearly with throttle
2. There is some engine resistance factor that is not shown in the equations, that works against the torque delivered by the engine to keep the engine rotating at a constant speed.
It seems as if all this should be pretty straight forward, since the books don't really bother all that much with the engine and drive train, but instead focuses on the tyres and suspension dynamics.
A pointer in any direction at all would be greatly appreciated!





RE: Engine throttle/resistance question
The change in torque is not linear with throttle position, and in fact is far from it. It will be somewhat closer to linear if you compare manifold pressure to torque, but this is still not linear. The proportional difference between throttle position and torque will be dependant on quite a few variables, such as rpm, cam timing, ignition timing, fuel properties, throttle plate size, plenum volume and shape, air quality, inlet manifold runner length and cross sectional area to name just a few.
For a particular engine, it is probably possible to get repeatable data for engine output vs manifold vacuum if the fuel and engine management system are controlled, and corrections are made for air quality.
The idle will be set by 1 of 2 methods.
The Engine Management System will control the throttle plate or a throttle bypass air bleed to control idle speed with it's computer.
The throttle plate or idle bypass will be adjusted until the idle speed is acceptable. The engine will then run at a speed where resistance and torque power are in equilibrium. If the supply of air or the resistance change, the idle speed will change to maintain equilibrium.
Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Engine throttle/resistance question
Typically an engine runs a MAP of about -70 kPA at idle, or, if you want to think of it that way, 1/3 of the WOT indicated power (or torque) at that speed.
However, the throttle butterfly will be barely cracked at that condition, to such an extent that a bypass is usually used instead.
From this 1/3 figure you can work out the frictional losses in the engine, which is why it does not continually speed up. The friction power required will probably rise proportional to engine speed^(just over 1)
Cheers
Greg Locock
RE: Engine throttle/resistance question
Lets not forget the increased motoring power loss of the engine at closed throttle. This can be a significant amount of load, easily exceeding the driveline and radiant loss at low speeds. Visualize pulling six 75mm suction cups 90mm, five times per second.
Franz
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Engine throttle/resistance question
Wow
Cheers
Greg Locock
RE: Engine throttle/resistance question
First of all, there are too many factors to take into consideration than to just model an engine of a particular car from some information on a website giving details such as max power and torque at certain rpm values, number of cylinders, cylinder capacities, etc.
Second of all, the resultant or net torque at the output of the engine is zero, since the engine management system ensures that the engine doesn't stall. If there were a resultant torque, the engine would keep on accelerating, since Torque=Inertia*Alpha ==>> Alpha=Torque/Inertia.
Thirdly, if the car is in neutral and I step on the throttle partway, the rpm value eventually stabilises on some arbitrary value and doesn't keep on accelerating to the redline due to the friction power rising to rpm^(>1). (If you can maybe supply an equation I would appreciate it.)
Fourthly, since I don't have all the detailed information concerning the engine, would it be, from a simulation point of view, a bad idea to assume the throttle to be linear? How badly would that affect my simulation?
That is quite a lot of information I need just to simulate a car in software. Can I just create the following test scenario:
Let’s say my car (engine) is idling at about 800 RPM, with a max torque of 160 N.m at that engine speed. I know I read somewhere during idling the typical engine produces about 50 N.m of torque (what a really bad assumption, I know, but I don’t have anything else to use), so the resistive forces of the engine must be generating -50 N.m of torque, or equal to the idling engine torque to bring the system in equilibrium. Suddenly I push the throttle to the max, thus the engine now delivers 160-50=110 N.m resultant torque to accelerate the car. This torque goes to the clutch->transmission->driveshaft->axle(s)->wheels, which eventually accelerates the car. Is this more or less accurate? Also, if I rev the engine up to some higher rpm value, does the engine management system see that the engine isn’t going to stall at that rpm value, and consequently doesn’t add any input to the engine? The engine then decelerates due to the internal engine friction, until the rpm value is low enough that the engine management system kicks in again and supplies input to the engine?
If you can maybe recommend a good book or informative websites that give more detailed information about engines (from an engineering perspective), I would greatly appreciate it! I hate having to read how some people who created car simulators fudged this and that just to get it to work more or less.
To Greg:
What are all the parameters you used in that equation? I gather that the 900/60 gives you the revolutions per second.
Thanks again for your help!
RE: Engine throttle/resistance question
By all means assume a linear relationship for throttle vs torque, for a racing game it should make very little difference. The characteristic is heavily non linear in real cars anyway.
Your model of the system is slightly over complex, you are treating it as a governed system such as a diesel. An SI engine runs open loop, so far as demand and output speed/torque goes.
Cheers
Greg Locock
RE: Engine throttle/resistance question
It would help us if we knew what you expected to learn from the software (or your customers) once its in use.
There is torque available from an engine at idle, just put it in gear and let out your clutch with no throttle (even a non-computer controlled engine), the car will move, although engine speed will drop.
RE: Engine throttle/resistance question
I would like to have several C++ classes that, in the end, describe a car in terms of engine, suspension, wheels/tyres, and steering. Obviously it’s going to take a fairly large amount of work just to get all the information about a specific type of car read in. Eventually, you're supposed to be able to drive the car around that you just created by means of a large amount of parameters. I don't what to unnecessarily fudge certain aspects of the car (as is done in almost all driving games), since we create accurate simulators where I work, not racing games or similar. I'm just having a hard time finding the required information in the text books we have in our library, and the Internet isn't all that useful either in terms of accurate car models. The only software that have accurate models of engines/cars are commercial products.
I don’t think that there is torque available at the engine at idle. I’m pretty sure of this, because I emailed the author of Motor Vehicle Dynamics, and he said that at idle the engine gives exactly enough power to overcome internal losses in the engine. If not, the engine would just keep on accelerating (Torque=Inertia*Alpha)..
I’ve got my virtual engine more or less up and running now; it’s idling correctly, drops back to the correct idling frequency if it was revved a bit, and delivers the correct output torque at the axles after multiplying with gear and final drive ratios as well. So far so good. I’ve also got the engine placed in a preliminary software car model/class. Most of the required parameters are hard coded for testing purposes at the moment, but the engine is causing the car to accelerate forward, and I can get the correct engine speed from the wheel rotational velocity. My next objective is to properly implement the tyres, suspension, and steering. So I’m busy reading up on that. It seems that most the books go into very great detail about tyres, and just ignore the details of the engine. If you can maybe recommend a good book on the dynamics of car engines, it might help me a lot.
RE: Engine throttle/resistance question
Define "torque available at the engine at idle" more clearly for a more accurate answer. At idle, the engine is still driving accessories, pumps, at least part of the transmission, etc. This means that there is necessarily some torque output beyond what is required to keep the engine alone spinning, otherwise the equation you mentioned above will tell you that the engine decelerates and stops.
RE: Engine throttle/resistance question
Ugly long sentence, reparse it at your leisure!
Cheers
Greg Locock
RE: Engine throttle/resistance question
You have to excuse the poor way at which I describe what I mean. I'm a computer engineer who doesn’t know all that much about cars and now suddenly I have to know a lot. Any of you guys know anything about a good textbook giving more detailed information about car engines and drive trains? Come on all you mechanical and automotive engineers! You had to learn all this somewhere?
Thanks for the replies! I greatly appreciate this!
RE: Engine throttle/resistance question
For instance at a high starting angle (a plate set at 30 degrees less then 90) the throttle response will be very quick. Because of its large permiter very little movement is needed to unviel a large flow area. So 10 percent of throttle may give you 40% of the engines maximium power.
Larger bores increase throttle response because they flow more at a lower throttle angle like the higher angle plates. However they do not make complete use of the entire throttle body. Once the engine has achieved an air velocity low enough not to cause too many parasitic losses over the throttle plate there is very little to be gained. In some cases power is even lost at WOT over 98 or 99 percent of maximium throttle angle.
It wouldnt be that hard to design a simulation that is static. Some simple dyno runs on your own car might allude to some realistic and generally acceptable potentiometer lines. Map sensors and 02 sensors as well as the torque sensing equipment would give you accurate values in MPG and part throttle performance and acceleration. Some simple timed tests would allude to its aerdynamics and mechanical losses. GTEC units mounted in the front and rear will give good lateral grip lines coupled with a regimen of traction testing involving differing kinds of corners and road cambers.
But building a simulation that is dynamic and responds to individual changes to a vehicle would nearly be impossible. The shear number of inputs required to make a vehicle behave realistically would overwelm even the largest computers.
-Travis-
RE: Engine throttle/resistance question
all the Source Code i've seen or come across at various sites , just use exactly like what you 1st were going to do
50 % Throttle = 50 % Torque
its sure the easiest method to use, i doubt if anyone would notice that your Simulation wasn't exactly mimicking
real-world results ???
Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3
RE: Engine throttle/resistance question
http://www.gamedev.net/reference/list.asp?categoryid=28
Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3
RE: Engine throttle/resistance question
Thanx for all your help, you guys have been really helpful!
RE: Engine throttle/resistance question
(general n/a tuning Carb and EFI)
Corky bell`s maximium boost
(Turbo tuning)
Corky bell`s superchargers
(Supercharger tuning)
Those are all good basic titles that cover all the spectrum of performance engines.
RE: Engine throttle/resistance question
RE: Engine throttle/resistance question
Fundamentals of engine design are covered by two excellent books, Charles Lafayette Taylor and Heywood. They are both called something obvious, the internal combustion engine, I think.
Cheers
Greg Locock
RE: Engine throttle/resistance question
One way to look at this, assuming stoichometric combustion (which isn't always the case) is that the power output of the engine will be proportional to the volume of air flow through the engine. The more the throttle is open and the higher the pressure in the intake manifold, the higher the engine power. The higher the engine RPM (within some limits), the more air will be pumped and the higher the power output of the engine.
Consider when the intake manifold pressure is one-half atmosphere and the engine speed is at 3000 rpm. The same amount of air (again, I'm ignoring all that stuff about valve timings, volumetric efficiency, etc just to make some rough calculations) will flow as if the engine rpm is 1500 rpm and the throttle is wide open (intake manifold pressure is one atmosphere).
Since Torque is equal to (engine power)/rpm, if we increase the engine rpm while keeping engine power constant, we will decrease the torque. In our example, the engine will have one-half the torque at 3000 rpm that it does at 1500 rpm. If we had a perfect gear set, we could reduce the speed of the shaft from 3000 rpm to 1500 rpm and. . .surprise!! The torque is now twice as much.
But. . . . .things aren't quite that simple in River City. Consider the amount of work the engine does to pump air through the engine. If the intake manifold pressure is constant, the amount of work to create the vacuum will be proportional to the mass of air pumped times the intake manifold pressure. In our example, the amount of air pumped is the same at both 1500 and 3000 rpm. However, the vacuum that the engine works against at 1500 rpm is one atmosphere; at 3000 rpm, it's only 0.5 atms. The pressure difference for the piston for the first case will be 0 atms; for the second case, 0.5 atms. It should be intuitively obvious to the most casual observer that the amount of vacuum work will be greater for the engine going at 3000 rpm than it will be for the engine going at 1500 rpm.
It is reasonably easy to show that amount of vacuum work for an engine with an intake manifold pressure of 0.25 atms is about 10% of a gas engine's power output. As engine speed increases (if the power is held constant) the amount of vacuum work will increase -- similar to the manner in which the amount of friction work will increase.
So you need to also consider the effect of intake manifold pressure on the vacuum work.
Hope this has raised the level of confusion!!!
RE: Engine throttle/resistance question
Dist Time Speed Accel Engine Gear
Feet ET MPH GForce RPM Number
12.0in -.212 6.343 1.364 4500 1
60 = 1.525 43.302 0.793 4500 1
330 = 4.424 79.265 0.395 4500 2
660 = 6.938 97.659 0.353 5477 2
1000 = 9.095 115.119 0.305 6321 2
1320 = 10.900 125.202 0.256 6915 2
---------------------------------------------
50 % PerCent Torque and HP Engine output
268.5 HP and 220 Torque
Dist Time Speed Accel Engine Gear
Feet ET MPH GForce RPM Number
12.0in -.271 4.959 0.834 4500 1
60 = 2.022 33.000 0.497 4500 1
330 = 5.768 63.073 0.308 6353 1
660 = 8.921 77.031 0.177 4500 2
1000 = 11.707 88.073 0.155 4859 2
1320 = 14.075 95.173 0.147 5286 2
thats from 10.900 seconds to 14.075 seconds of time
if you were simulating much quicker transitions
in distances & time , on/off throttle or
throttle position changes during a Road Course Race,
it would seem the ;
50 % Throttle Position = 50 % Torque or HP
would work pretty good ??
Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3
RE: Engine throttle/resistance question
The throttle position is not clearly defined as to whether it is throttle plate angle, or pedal travel position. If it is pedal travel, it is complicated even more by the mechanical linkage often being very non linear to throttle angle
Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Engine throttle/resistance question
Is it the case that the mathematics of the whole engine are so complicated that to mathematically simulate it, you will not be able to outperform actually measuring it.
There are few user inputs: basically accelerator position, unless you want to get into detail of things running differently when the lights are on, heating on, etc. And there are few outputs: basical engine torque. There is a big factor feeding back internally, RPM.
I'd be tempted to make a table of accelerator positions and RPM, and store output torque values, or something like that. You could fill the table with computed or measured values, and I think you'd find computed values never quite agree with measure values no matter how much time you put into your software model.
How the engine behaves will depend on valve timing, throttle, bore, stroke, exhaust system, and lots of other things.
Is it really worth producing a precise software model based on these factors for a system which has basically one input (accelerator position) and one output (a drive shaft).
Does your software model really need all the extra detail, for example to calculate when a car would run out of petrol, to know how much the battery is charging, etc?
Given that you are looking at suspension, etc too, I would guess that you are really trying to model a car's behaviour rather than the engine's. It seems to me that engine modelling via precise simulation is only really useful for trying to make better engines, and that car modelling is maybe far more accurately done by looking up stored real life engine data.
These are personal opinions based on what I have read and trying to understand the real task at hand, and are not meant to reflect on any choices you or your employer has already made. I am just looking at what I have read from a software engineering point of view, rather than a solely mechanical point of view.
The simulation from accelerator position to output torque seems to me to be too complicated to attack from fundamentals - how does the visosity of air in the exhaust pipe change with temperature, or whatever else might be used in the perfect software model of a car. Do you really WANT to worry about how much engine power is sapped by pulling air past the butterfly and intake valves based on the sizes of various orifices and how far up in its stroke the piston gets before the intake valve closes, etc?
If you use a look-up table approach, you might be able to use the same software for a normally asperated petrol engine and a supercharged diesel engine, or whatever. If you simulate things, you may have to start again when you worry about diesels varying fuel intake instead of petrol engines regulating air (and fuel mixture) intake. Presumably a fuel cell vehicle's "engine" has clearly defined amounts of torque available for different accelerator positions.
I hope I don't get into trouble on this forum for asking whether you really want to solve your problem rather than actually helping you solve it!
RE: Engine throttle/resistance question
First of all, thanx for your post. Yes, I do know about torque or power tables, and that is how I would prefer to model the car/engine. I also have some equations that would build you a torque table if you only have the max power and/or torque at specified rpm values, and no, they never match the actual values. Currently you have the option of supplying the torque table yourself, or generating it from the maximum torque or power at a certain RPM value.
What I am currently doing is that I already have a torque table available for the engine, as well as gear ratios, gear inertias, engine inertia, final drive ratio, final drive inertia, gear efficiencies, final drive efficiency, and wheel inertia (and obviously also wheel radius). I then assume the accelerator pedal position is linear with the output torque, thus if in my torque table I have for instance 300 N.m torque at 3000 RPM, then I would assume I have 150 N.m torque at 3000 RPM with my accelerator pedal pushed only half-way in (50%). I then calculate the torque delivered to the wheels by means of the gear ratios and final drive ratio, and finally I get the force delivered to the road surface by the wheels. I then update my RPM value of my engine by means of the speed at which my wheels are rotating (unless my car is in neutral). This process continues indefinitely, and it seems to work ok, although I have some bugs I need to fix… as always.
I obviously have to model the engine behaviour somewhat accurately, but not up to the point of insanity. I’m having a hard time getting my engine to idle correctly without fudging it too much. I made my own internal resistance function, which you can design for specific losses at specific engine speeds, but I REALLY DON’T like doing that! I guess I have to model the clutch as well, but currently my model just refuses to stall, and keeps on pushing the engine up to the minimum rpm value until the wheels start moving at the correct velocity to generate the desired RPM value. This naturally causes the tyres to spin, so it’s all very ugly at the moment.
Modelling a car is far harder than I ever thought it would be, and I haven’t even gotten to the tyre and suspension part yet! At least I am enjoying it, and one only learns by struggling. A lot.
Riaan Nieuwoudt
Computer Engineer
riaan.nieuwoudt@5dt.com
RE: Engine throttle/resistance question
I'd try actually driving some cars to see how they react. Every car I've ever driven has a very noticeably nonlinear throttle response. Luxury cars tend to be very sluggish until you floor them, whereas some sporty cars feels like >70% of engine torque is available with maybe 1/4 of the pedal depressed. Older cars with carburetors sometimes had very sudden changes in torque output when the load-sensitive secondaries opened up.
Another thing to consider is that some fuel delivery systems have a very noticeable delay between throttle opening and engine response. This can be important. Sometimes the speed at which you push down the gas pedal also has a noticeable effect in a real car.
As for getting a feel for the internal losses, you might want to try putting a car in neutral and revving it up. It takes a good amount of throttle to reach redline even with no load on the engine.
When spinning the tires when trying to stall the car, I think that you should check if your inertias are way off and the clutch engages too quickly.
RE: Engine throttle/resistance question
Sounds like you might be trying to re-invent the wheel.
There is a mathematical model of an engine/vehicle in the public domain. It is called ADVISOR (the last free version was 2002), runs under Matlab/Simulink and was developed to model hybrid cars by the US DOE. If you download a copy of this, you could interrogate the maths behind the various system models.
The commercial rights were purchased by AVL in 2003, but previous versions should still be around.
Andy
RE: Engine throttle/resistance question
If we look at the red torque curve here, we can see that it drops and will eventually cross zero at around 3800 RPM. Another thing to note is that it is labelled net torque, from which we can suppose that there is a notion of gross torque: the torque the engine produces but some of it being lost for example due to the pistons rubbing in the cylinders.
If a simple software engine was to take the net torque figures for the Ford and scale them for accelerator considerations, (for example at 3500 RPM with 10% accelerator it might assume 50 units of torque insead of 500), then the graph would have the same shape, just smaller. It would still cross the horizontal zero line at 3800 RPM.
In neutral, speeds above 3800 RPM would produce negative net torque and the engine would slow down. Speeds below 3800 RPM would produce positive net torque and the engine would speed up. So in both cases (light and heavy accelerator), the engine would stabilize at 3800 RPM.
But let's say (for the sake of argument) that the 500 units of net torque (at 3500 RPM on the graph) is really 550 units of torque from the engine, with 50 units being lost internally due to pistons rubbing, etc. It is not right to scale the 50 units of piston rubbing when considering light accelerator operation. So we shouldn't calculate 10%*500=50 units of torque at 10% accelerator, but maybe 10% of 550=55 units of engine torque, and still subtract 50 units of piston friction, etc. That would give just 5 units of net torque (almost zero) at 3500 RPM, and we can see that at 10% accelerator the engine would stabilize somewhere near 3500 RPM instead of 3800.
At 5% accelerator we'd have about (5%*550)-50 unit of torque ie significant engine braking and the engine speed would slow down if at 3500 RPM.
So I think all that is need to make a software car engine stall, provide engine braking, stabilise at high RPM under heavy accelerator and at low RPM under light accelerator is a crude model for engine losses in general, eg piston friction and for at least some of these losses not to be scaled for different accelerator settings.
The net torque curve crossing zero is surely what causes an engine speed to stabilize, and any linear scaling of this curve will just cause the engine to idle at the same RPM, eg 3800 for the graph shown.
But as shown, even a crude approximation of some engine loss not scaled for fuel usage (eg piston rubbing depending basically on engine speed rather than load) will provide engine braking, lower idling speeds and I guess the ability to stall your software engine too.
I guess what I'm saying is a software model like this
torque=accelarator*grosstorquegraph(RPM)-losses(RPM)
will perform much more realistically than one like this
torque=accelerator*nettorquegraph(RPM)
==
So in your first posting you said
I further assume the torque is scaled linearly with the throttle, thus if the pedal is pushed only half way, you only get 50% torque at the specified rpm value, etc. If this is incorrect, please inform me.
Incorrect.
Even if you try to account for the accelerator behaviour being non-linear with something like this
torque=nonlinearacceleratorfunction(accelerator)*nettorquegraph(RPM)
you are still going to have problems with
nettorquegraph(3800) being zero and that would be you idling speed.
I'm sure putting losses in your model a bit like shown will make it behave like a car, even getting it to behave like the right car remains a bit elusive.
RE: Engine throttle/resistance question
Yes, this is the way I have been modeling it thusfar, with
OutputTorque = Accelerator*GrossTorque(RPM)-Losses(RPM)
I assume then that the loss is also a function of the engine speed? I also assume that the losses increase with RPM value, although not by much? I have made the assumption that at idle
Accelerator@Idle*GrossTorque(RPM@Idle) = Losses(RPM@Idle)?
This makes sense for me, since the engine isn't accelerating or decelerating at idle? Or could it be that the engine management system is keeping the engine alive?
Riaan Nieuwoudt
Computer Engineer
riaan.nieuwoudt@5dt.com
RE: Engine throttle/resistance question
Engines idled long before there were management systems. I'd forget engine management completely until you have a satifactorily running car simulation.
I'm sure it has to be a net-torque curve falling with RPM that crosses zero that causes idling.
If the engine speed is lower than idling, torque would be positive and the engine would pick up. If the engine speed is above idling speed then net torque would be negative and the engine would slow down.
Hence the system would stabilize at idling speed.
I wasn't satisfied with what I wrote yesterday and thought some more about it. I'm going to discuss pertol engines rather than diesels engines, as that is what I think I understand best.
Yesterday I showed that engine stabilization speed could be lowered with lower accelerator if some engine losses were included in a software model and independent of accelerator settings, and I used piston rubbing as an example of something fairly independent of accelerator (for a given RPM).
(In contrast I said that simply scaling a torque curve for
accelerator considerations, eg
torque=nonlinearacceleratorfunction(accelerator)*nettorquegraph(RPM)
would still lead to the simulated engine wanting to idle at 3800 since nettorquegraph(3800) is zero.)
I need to go, so I'm going to have to make this quick.
The engine gives it best torque roughly where it is getting the most air into the cylinders; there is a whole question of engine breathing and surrounding complexities which make airflow a critical factor in the engine's operation. Other things being equal, if you made the engine capacity a bit bigger it will produce peak torque at a lower RPM, ie with a similar airflow.
If we turn that argument on its head, restricting airflow (with a throttle) means that the engine would produce its torque peak at a lower RPM.
So with lower airflow, the torque peak would move to lower RPM.
I really have to go, people are impatient
I'll write more later, but for less accelerator something like this might be worth trying
torque = accelerator*grosstorquefunction(RPM/accelerator) – losses (RPM)
Buy 4 now
RE: Engine throttle/resistance question
Okay, just to tidy up a loose end, let me explain.
If we had a function in the computer which produced a line similar to the red line in the picture, then there would be no problem making the software version of a car engine 'idle' at 3800 RPM.
With a throttle partly activated, and restricting airflow into the cylinders, the engine (in the picture) would have problems breathing at 2000 RPM where it makes its peak torque in normal operation. The torque curve might have a similar shape, but be scaled so that peak torque (at part throttle) came earlier, and if we make things as simple as possible, we might assume peak torque at 1500 RPM for 75% throttle: just a plain linear scaling. So torque at 1500 RPM at 75% throttle would be 'like' torque at 2000 RPM for full throttle. So we'd want to use a figure from the torque curve for 2000 RPM if we were at 1500 RPM. Ie we'd need to look at the torque function for (1500/0.75) to get a figure to use for torque at 1500 with 75% throttle.
Generalizing that, to account for the shape of the torque curve moving to lower RPM at lower accelerator settings, I'd try something like grosstorquefunction(RPM/accelerator) to give the curve its torque curve shape, and scale it in some way, as you are clearly going to get less torque with less air in.
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Obviously it would be better if multidimensional data for an engine were available and the software could just look up net torque for given RPM and accelerator settings, or something like that.
But I'm still working on the assumption that behaving like a car is pretty good for your application and we are not trying to work out exact engine behaviour for a particular engine.
I could show you the torque curve for a completely software simulated engine at wide open throttle. It calculates pressures and temperatures using gas laws, and it considers volumetric efficiency, losses from compressing the intake charge, reversing the pistion inertia, and things like that. The final result is still nevertheless not a very convincing torque curve.
From this conversation I'm beginning to see why!
By the way, what I write is all open to debate. I'm looking at this as an area that interests me and not as my speciality - whatever that is! I'd welcome it if somebody knowledgable stepped in with a simple expanation of a good
software model of an engine's behaviour.
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By the way, your assumptions seem reasonable to me. You should be able to use them to help you set up your software car to idle at a particular RPM and to determine what to set as a range for the accelerator, eg 20% to 100% or whatever.
I just did a quick search on the internet.
http://www.f150online.com/forums/archive/topic/157...
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Typical calculated engine load (which in Fords is a ve calculation - volumetric efficiency) at *idle* is around 25% in these trucks for example.
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Throttle position is going to be about 20% at idle - not zero as some might assume.
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3800*0.25 is 950 and 3800*0.2 is 760. If your software engine idled around those sort of speeds it would be behaving too differently to my car.
I'm going to email you an article which shows a torque curve shifting in relation to RPM as a result of engine breathing considerations so that you can make critical judgements about whether there is justification for what I've said.
RE: Engine throttle/resistance question
if you had enough info about the engine you could build the torque table from a simulation at wot (i use an old version of desktop dyno) and then change the cfm rating of the intake system to represent the throttle position.
example (using very made up numbers)
500 cfm induction at 3000 rpm is 200 hp (wot)
with throttle open part way
200 cfm induction at 3000 rpm is 75 hp
cfm= cubic feet per minute of airflow
cfm is measured ot a specific vaccume drop to rate the flow Im sure there has to be a way to find out the number from throttle bore and throttle plate angle as it is a fairly straightforeward physical property.