Here's an example of my table for different gradients at speed 10km/h.

Hi,

The physics is on your side, i.e. the calculation is OK. I would do the same. The proof comes from the comparison the uphill and downhill values where uphill is bigger and the difference increase with slope.

Can You show the table with your 120km/h data. This would help to understand where the problem is arising.

Cheers,

Jarek

Hi Jarek, thank you for your reply.

Here is the table for 120km/h. I think the issue is when I divide it with the speed; when divided by 10 (km/h) = higher g/km emissions than when divided by 120. Because in g/s I have 1,458 for 10km/h and way more for 120km/h (7,165 g/s). These values seem correct, because common sense tells me that at a very low speed going downhill there will be less fuel consumption (therefore less emissions) than on a steep uphill road at 120km/h. I'm unsure if I am somehow "cancelling" the speed I used in the VSP calculation when doing this last bit of calculation and that may be causing the issue...

Thank you for your help.

Tanya

Hi Tanya,

The numbers are not laying but maybe the measurement are. The calculation is correct. From my point of view the results are like expected.

Look, when you are driving fast cars consume more fuel mainly due to the air friction (viscosity). Not because of the performance of the engine itself. Using units g/km You express the amount of fuel consumed by the engine in time it needs to pass the distance of 1km. With speed 10km/h it takes 360 sec (6min). But with 120 km/h its only half a minute (30 sec). So to get the same result Your engine would need to burn 12 time more gasoline. Your results suggest only 3.

I could understand it if it is performance of the engine (not the vehicle). I'm not specialist but i would expect that 10km/h are not preferable velocity for most of the vehicles and engines works in non perfect rpm. I can image when You will make the graph of dependency of fuel consumption vs velocity - there will be some minimum value visible at the plot and it will not be in low values of velocity.

Your common sense may be affected by the fact that air viscosity drag force is increasing with square of velocity so the cars going fast consume more fuel than when driven slowly.

But also this is not true anymore. Car going with a speed of 10km/h will consume much more gasoline than going with 90km/h what is usually most economic velocity. 120 is also quite OK. At least for all cars i had by now. In Poland, where i live we use unit L/100km so I have quite good feeling of this unit.

I hope it helps a bit - let me know Your opinion. I would also like to know more about your experiment.

Pozdrowienia serdeczne (pol. cordial greetings),

Jarek

Hello Jarek.

Thank you for your kind reply once again. Sorry for the delay.

I'm not al all "fluent" in physics or mechanics. What I've tried to achieve was by reading A LOT of papers, which gave me a teeny tiny sense of knowledge in what we're discussing and the minimum tools to apply it. That being said, I apologise if I say something foolish.

I'm trying to measure CO2 emissions from different driving conditions, not fuel consumption; although, from what I've gathered, it seems they are proportional to one another; that is, more fuel consumption, more emissions.

I get what you said with the relationship between the amount of fuel consumed in time and it makes sense to me, but I still think that for 10km/h, the value is extremely high, ate least for downhill slopes. For example, when I take my kids to school, there's this portion of the trip that I have to pass in the middle of the city and it's a rather long downhill slope (not even that hilly). I can easily go 10 or 20km/h and maintain that speed without using the pedal to increase the speed or to lower it (breaking). I know this is not scientific at all, but this, to me, seems like that, although there is some fuel consumption because the engine is on, fuel consumption (or CO2 emissions) will be low because there is minimum effort put on the engine to move the vehicle. I find it hard to believe it will be in the order of 524,88 g/km. But maybe I'm spectacularly wrong! :)

As for my work, I'm doing my PhD thesis and am creating an agent-based model coupled with GIS to determine how different policy changes can affect CO2 emissions made by passenger cars in my study area. Because the area has 1/2 and 1/2 flat:hilly areas, I decided to make it harder for myself and include speed/slope/emissions information in my model.

I really appreciate the help you've given. At least I know I got the math right, which was also one of my major worries. I guess I'll try to work around this stubbornness of mine in some way. It would be great to use "real" values as input for my model; still, I guess the most important thing is getting the model right. Parameters can easily be changes any time, right?

Thank you once again,

Tanya

Hi Tanya,

I agree with Your point - when driving downhill with normal passenger car the fuel consumption decrease to 0. And I would expect that the results will agree with it. But the experimental data might come from the experiment which was conducted in other conditions. E.g. slope might be modeled on the closed pure laboratory experimental unit, where the engine is operating constantly.

That is why i would suggest to look more skeptical on data source than Your unit recalculations. I also do not trust such results.

Pozdrowienia serdeczne (cordial greetings)

Jarek

I agree Jarek. I've been looking at other potential sources of information. I'm finally what I think is the right track. Time will tell. Nevertheless, thank you for this brainstorm session. It is nice to have a critical eye with a fresh point of view to help me improve my work. You have been most helpful.

All the best.