For constant-speed, level road driving, the real-time MPG indictor tells you exactly what you need to know. You can compare using differnet gears and throttle positions and know which is the best choice.
But for accelerating and hill climbing, it can be hard to tell what's best. If you are accelerating hard, the bar is down at 25 mpg or so, which seems bad, but in fact the engine may be operating very efficiently--using a lot of energy but storing the result as kinetic energy of the car. You may be better off opening the throttle pretty wide (where the engine is efficient), accelerating quickly, and then suddenly leveling off your speed, rather than gently sneaking up on the desired speed, even though the latter looks better on the indicator.
My purpose isn't to start at debate about acceration techniques (and if you want to have one, I suggest starting, or picking up a thread in the MPG Issues forum). Rather, I want to suggest an indicator that is corrected for acceleration and slopes. Then, rather than debating techniques, we can learn what works best by watching the indicator.
So how do we sense and correct for slopes and acceleration? Actually, an accelerometer can't tell the difference between accelaration and slope (nor can an inclinometer), which sounds like it might be a problem, except that it turns out that for the purpose of what I'm suggesting here, we don't need to know which it is. Either way, we are storing energy. What I want to know is whether the bar has dipped to 25 mpg because I'm using the engine in a very inefficient regime, or because I'm storing most of the energy coming out of it.
Here's the algorithm to do it: From the acceleration and the speed, one can easily calculate the rate at which energy is being stored--the power going into energy storage. The hard part is figuring out how to correct the raw mpg number given that number. What I'd suggest is to calculate how many more miles you could theoretically go (at the present speed) with the extra energy you are storing.
The IMA energy storage should also factor in, but let's set that aside for now.
To be mathematically specific, the indicator now displays
MPG = velocity (mph)/(gas flow rate (gallons per hour))
That can be re-written as
MPG = (velocity/power)*(power/gas flow) = (velocity/power)*(efficiency)
What I'd like to display instead is
MPG_effective
= ( velocity/(power for steady speed) )*(efficiency of engine now)
= ( velocity/(power for steady speed) )*(power now/gas flow)
To get power for steady speed, I suggest that data be stored in a table. That data could come from a roll-down test--record speed as a function of time on a level road, while coasting in neutral. Or it could come from calculations using the published Cd and A, and an estimate of tire rolling resitance. If it comes from the roll-down test, power (from F = ma) is m*a*v where a is velocity and a is acceleration.
Using the fact that total power now is the power going into accelertion, plus the power needed to maintain speed:
MPG_effective
= ( v/(m*|a_rolldown|*v) )*(m*(a_now+|a_rolldown|)*v)/gas flow)
= [(a_now+|a_rolldown)/(|a_rolldown|)] * (v)/(gas flow)
= [(a_now+|a_rolldown|)/(|a_rolldown|)] * (actual present MPG)
That's a nice simple result.
So now now can that be implemented? How can one get the actual present MPG data? In fact, how does the FCD sense the fuel consumption? Can one tap into that sensor? Is that data would be available on OBDII? I got a palm-pilot based OBDII scanner (Auterra Dyno-Scan) that is supposed to be able to display real-time or trip MPG for people who don't have a FCD, but that feature doesn't seem to work on an Insight.
If all else fails, an option is optical sensing of the FCD bargraph, or just making a unit that would display the factor [(a_now+a_rolldown)/(a_rolldown)], and the driver could do mental arithmetic to calculate the result--that way, if you are driving your Insight instead of walking or riding a bike, you'd still get mental exercise, even if not any physical exercise!
One cool thing about this formula is that the result goes negative when you brake. That's a nice way to show the negative effect of braking on mpg--something else that doesn't show up in the present system. (I'm again ignoring the effect of IMA and regenerative braking)
Charlie
But for accelerating and hill climbing, it can be hard to tell what's best. If you are accelerating hard, the bar is down at 25 mpg or so, which seems bad, but in fact the engine may be operating very efficiently--using a lot of energy but storing the result as kinetic energy of the car. You may be better off opening the throttle pretty wide (where the engine is efficient), accelerating quickly, and then suddenly leveling off your speed, rather than gently sneaking up on the desired speed, even though the latter looks better on the indicator.
My purpose isn't to start at debate about acceration techniques (and if you want to have one, I suggest starting, or picking up a thread in the MPG Issues forum). Rather, I want to suggest an indicator that is corrected for acceleration and slopes. Then, rather than debating techniques, we can learn what works best by watching the indicator.
So how do we sense and correct for slopes and acceleration? Actually, an accelerometer can't tell the difference between accelaration and slope (nor can an inclinometer), which sounds like it might be a problem, except that it turns out that for the purpose of what I'm suggesting here, we don't need to know which it is. Either way, we are storing energy. What I want to know is whether the bar has dipped to 25 mpg because I'm using the engine in a very inefficient regime, or because I'm storing most of the energy coming out of it.
Here's the algorithm to do it: From the acceleration and the speed, one can easily calculate the rate at which energy is being stored--the power going into energy storage. The hard part is figuring out how to correct the raw mpg number given that number. What I'd suggest is to calculate how many more miles you could theoretically go (at the present speed) with the extra energy you are storing.
The IMA energy storage should also factor in, but let's set that aside for now.
To be mathematically specific, the indicator now displays
MPG = velocity (mph)/(gas flow rate (gallons per hour))
That can be re-written as
MPG = (velocity/power)*(power/gas flow) = (velocity/power)*(efficiency)
What I'd like to display instead is
MPG_effective
= ( velocity/(power for steady speed) )*(efficiency of engine now)
= ( velocity/(power for steady speed) )*(power now/gas flow)
To get power for steady speed, I suggest that data be stored in a table. That data could come from a roll-down test--record speed as a function of time on a level road, while coasting in neutral. Or it could come from calculations using the published Cd and A, and an estimate of tire rolling resitance. If it comes from the roll-down test, power (from F = ma) is m*a*v where a is velocity and a is acceleration.
Using the fact that total power now is the power going into accelertion, plus the power needed to maintain speed:
MPG_effective
= ( v/(m*|a_rolldown|*v) )*(m*(a_now+|a_rolldown|)*v)/gas flow)
= [(a_now+|a_rolldown)/(|a_rolldown|)] * (v)/(gas flow)
= [(a_now+|a_rolldown|)/(|a_rolldown|)] * (actual present MPG)
That's a nice simple result.
So now now can that be implemented? How can one get the actual present MPG data? In fact, how does the FCD sense the fuel consumption? Can one tap into that sensor? Is that data would be available on OBDII? I got a palm-pilot based OBDII scanner (Auterra Dyno-Scan) that is supposed to be able to display real-time or trip MPG for people who don't have a FCD, but that feature doesn't seem to work on an Insight.
If all else fails, an option is optical sensing of the FCD bargraph, or just making a unit that would display the factor [(a_now+a_rolldown)/(a_rolldown)], and the driver could do mental arithmetic to calculate the result--that way, if you are driving your Insight instead of walking or riding a bike, you'd still get mental exercise, even if not any physical exercise!
One cool thing about this formula is that the result goes negative when you brake. That's a nice way to show the negative effect of braking on mpg--something else that doesn't show up in the present system. (I'm again ignoring the effect of IMA and regenerative braking)
Charlie