Low FE using gas engine - is this normal?

[bigalpha]

I've noticed that I almost always get super low FE while using the gas motor unless I'm over 55-60.  If I'm cruising on flat ground at 45, the MPG meter shows ~20.  I would expect it to be higher - is this normal?

Edited August 6, 2013 by bigalpha

[GaryM]

_{I am not a Ford engineer, but I am an engineer with a very technical background. I have been observing the behavior of my car since I got it and I have a good grasp of what it is doing and I am really learning how to drive it to make the most of it in my situation.}

 

_{This poor MPG when the IC engine fires up is "normal" with how the C-Max seems to work.}

 

_{When you are running in EV mode, you are getting infinite MPG but all of the energy has to come from the gasoline at some point. It turns out that piston engines are much more efficient with a fair amount of load on the crank. An idling engine is wasting all the fuel just turning the internal drag of the engine. Up to some point, the more power the engine is making, the more efficient it is as the losses from internal drag do not increase much at all with the increase in load. So, what Ford seems to have done in the C-Max is to let you run the batteries down more than most other Hybrids, and then it works the engine harder into the generator to push charge into the HV battery while working the engine in a very efficient load point. The instant MPG while it is doing this does look bad, but when you average that fuel burn with the EV running it comes out very close to the 47 MPG rating.}

[StoBro2]

So, in essence, the car does all of the pulsing and gliding so you don't have to.

[bigalpha]

So if engines are more efficient under load, then shouldn't we be seeing > 20mpg while the engine is running and charging the battery?

[plus 3 golfer]

So if engines are more efficient under load, then shouldn't we be seeing > 20mpg while the engine is running and charging the battery?

The engine efficiency is measured by input energy divided by output energy. This is not the same as the fuel economy of the vehicle.  

 

During charging, part of the power from the engine drives the wheels and part charges the battery.   The algorithm that calculates FE divides miles traveled by total fuel used. So, during the time of "heavy" charging of the battery, the instantaneous FE will be very low because all fuel burned is included in the FE calculation (not just an estimate of the fuel used to propel the car).  When the battery is supplying all the power to drive the car, the instantaneous FE will be infinite because the engine is off (no fuel is being burned) and the FE gauge will show the blue 60+ mpg as an indication of infinite FE (distance traveled / zero fuel burned).  

 

Over the course of many charging cycles of the HV battery, the average FE will be higher than it otherwise would have been absent the charging because the engine was operating more efficiently during charging. For example, I posted a typical Brake Specific Fuel Consumption curve before (probably a diesel but the principle is the same) as shown below.  The left access (y axis) is torque (load) and the bottom axis (x axis) is rpm.  

 

As one can see the engine is more efficient (the curves on the graph show fuel used - lower number is better) when the load is higher (blue shaded areas on the graph).  During low speed cruising (say 1200 - 1500 rpm at low load, the BSFC (fuel use) would likely be in the green areas (300 - 500 grams / kWh).  But if charging brings the load up to the blue areas of the graph, the fuel use could be in the 240 - 260 grams / kWh.  This might be an average increase in fuel use efficiency of 37.5% = (400 - 250) / 400.  

 

But, there are losses (generator, inverter, HV battery, motor, wiring) to convert the fuel used for charging to drive the vehicle.  So if one assumes 1/2 the fuel used during charging was used to charge, 50 % of the 250 grams / kWh would be subject to the conversion efficiency of the electrical hybrid components (assume  80%).  Thus, the effective fuel used during charging ( the 250 number) becomes (50% * 250) +  (50% * 250 / 0.8) = 281.  So, the overall effective increase in fuel efficiency use might drop to 30% = (400 - 381) / 400.  So, if one would normally get 32 mpg running the gas engine only, the hybrid powertrain operation might improve the FE to 41.2 mpg  = (32 * 1.3).  Of course we don't know the actual BSFC of the C-Max engine, loads at various rpm, nor the conversion losses.  But this does show how charging, storing the energy, and using it later improves overall FE.

 

[JohnnOhio]

I see my instant mpg gauge run in the 20's while the ICE is warming up or on hills but never on flat open highway. 

[fotomoto]

So, in essence, the car does all of the pulsing and gliding so you don't have to.

 

Yes.......... and no.

 

Yes in the regard as to how GaryM so nicely explained in post #2.   

 

No in the regard that if you just drive it and don't "help" the car get into EV mode as much as possible in won't auto P&G.

 

Until cars are smart enough to know the terrain and traffic, auto P&G won't be effective.  Some cars all ready have adaptive cruise control that uses radar to recognize the speed of the car in front of them so that tech is nearly there.  Now if the car could also compute terrain (hills, down grades, etc), wind, recognize stop lights changing, etc.......    We human drivers are still important.    For now.......... <insert evil laugh here>

[plus 3 golfer]

I see my instant mpg gauge run in the 20's while the ICE is warming up or on hills but never on flat open highway. 

It depends on conditions.  Increase speed on flat roads or hills while charging and it can easily drop into the 20's or less.

[JAZ]

Plus 3, just because I 'liked' your post above doesn't necessarily mean I understand it.

[bigalpha]

So basically, the car FE meter doesn't account for the fact that the gas motor is propelling the car AND charging the battery; I didn't even think about that.  Thanks for explaining more of the process, I enjoyed getting into the technical details, Plus 3.

[plus 3 golfer]

So basically, the car FE meter doesn't account for the fact that the gas motor is propelling the car AND charging the battery; I didn't even think about that.  ...

Yes, just to be clear, the FE meter shows FE based on ALL fuel used by ICE instantaneously irrespective of why the fuel was burned by ICE.

[bigalpha]

Yes, just to be clear, the FE meter shows FE based on ALL fuel used by ICE instantaneously irrespective of why the fuel was burned by ICE.

 

Kind of related to this - how accurate do you think the car can measure how much fuel has been burned? 

[GaryM]

In theory, the amount of fuel burned should be very accurately known by the engine computer. Modern EFI fuel injectors work by flowing a very precise amount of fuel per second that the valve is open. They are pulsed for a few milliseconds for each engine cycle. Some engines fire all injectors every rotation, others fire each injector right before the intake stroke. Some newer engines inject directly into the cylinder, but it looks like the C-Max engine still injects into the intake port before the intake valve. So if an injector flows 120 cc/min of fuel, and it is open for 4 minutes in an hour of driving, it injected 480 cc of fuel in that hour. It is all math. These numbers are not guess work, at 2000 rpm, the engine turns one time in 30 ms and it takes about a 2 ms pulse to feed the fuel at cruise load. so the injector is open only 1/15 of the time. that is 4 minutes per hour. There is some small variance in the injectors and the fuel pressure, which will also change the flow rate, but it should hold very constant in any given car. Any variance is corrected for in the engine computer by checking the exhaust gas mixture. All of the ECU's I have worked on use a short term and long term fuel trim to adjust the mixture to account for injector variance as well as the energy content of fuel. Pure gasoline versus gasoline with 10% ethanol will require a different amount of fuel for the same engine load point. The big question is if Ford bothers to calibrate each car to read out the true flow rate of the injectors installed?? My guess is they do not bother as the design of modern fuel injectors is very accurate and if they fall out of range in testing, they will be rejected and not installed in a production car. Very few cars can tune each cylinder individually, so this makes it critical that the 4 injectors all match very closely to each other. It seems to me it would be easier for Ford to make sure all injectors are close enough to work in any random set than to pick sets of 4 that match. That would minimize how far off the true flow is from the calculated ideal.

[nsteblay]

Very interesting discussion.  I am curious ...

 

I rented a Prius last week.  One observation I had was the ICE kicks in about 30 MPH - yet I was seeing instant 50+ MPG while driving 65-70 MPH.  I think Toyota has nailed the Negative Split Mode so that the ICE is using very little fuel when cruising at high speeds.  This approach also seems to reduce the variability in mileage.  The downsides of the Toyota were very little power for acceleration and of course the other aesthetics related to the Prius.

 

I didn't notice a huge delay in getting to Negative Split Mode with the Toyota.  Why does the C-Max need a full battery charge to get to Negative Split Mode?  The time to get to full charge with ICE at high speeds really brings down the overall MPG average. 

[plus 3 golfer]

Very interesting discussion.  I am curious ...

 

I rented a Prius last week.  One observation I had was the ICE kicks in about 30 MPH - yet I was seeing instant 50+ MPG while driving 65-70 MPH.  I think Toyota has nailed the Negative Split Mode so that the ICE is using very little fuel when cruising at high speeds.  This approach also seems to reduce the variability in mileage.  The downsides of the Toyota were very little power for acceleration and of course the other aesthetics related to the Prius.

 

I didn't notice a huge delay in getting to Negative Split Mode with the Toyota.  Why does the C-Max need a full battery charge to get to Negative Split Mode?  The time to get to full charge with ICE at high speeds really brings down the overall MPG average. 

The reason the Prius gets higher FE than the C-Max at higher speeds is virtually all due to it's weight advantage and lower drag. A simple calculation will show that the nearly 600 pounds of additional weight of the C-Max and the additional C_d of  0.05 of the C-Max results in the C-Max requiring about 19% more power than the Prius at 65 -70 mph all else being the same.   Hence, the reason the Prius gets 8.2 mpg better FE at 65 mph (18.6% better} and 7.4 mpg better FE at 70 mph (19.4% better) than the C-Max. See cleanmpg.com test result graph below.

 

Hopefully, the PCM update will better optimize the C-Max hybrid powertrain operation and reduce this difference in FE between the Prius and the C-Max. 

 

 

  

[salsaguy]

don't forget that the cmax is a much heavier car, has more drag due to the body style, has more performance tires not eco tires compared to prius, but the biggest difference is the cmax uses a totally different battery technology (lithium ion, compared to old school prius nickel metal hydride nimh). they have to tweak their curves to max mpg for each factor of the car and how it works together.

the Prius can get 50 mpg but is lighter (not as solid in turns/performance driving), has no style, and is very old tech compared to the C-max.

 

 

Very interesting discussion.  I am curious ...

 

I rented a Prius last week.  One observation I had was the ICE kicks in about 30 MPH - yet I was seeing instant 50+ MPG while driving 65-70 MPH.  I think Toyota has nailed the Negative Split Mode so that the ICE is using very little fuel when cruising at high speeds.  This approach also seems to reduce the variability in mileage.  The downsides of the Toyota were very little power for acceleration and of course the other aesthetics related to the Prius.

 

I didn't notice a huge delay in getting to Negative Split Mode with the Toyota.  Why does the C-Max need a full battery charge to get to Negative Split Mode?  The time to get to full charge with ICE at high speeds really brings down the overall MPG average. 

Edited August 15, 2013 by salsaguy

[Jus-A-CMax]

So if I lose some weight...put on some Speedos.....I might get better FE....huh...never considered that...wow. stunning.

 

:stirpot: