HEV 4 Modes - Effects of Temperature

[nsteblay]

Image of C-Max HEV

 

https://docs.google.com/file/d/0B-dKQZnDd9FQNEx4UW0zQzJZZWM/edit?usp=sharing

 

4 Modes of Operation

 

Series Mode

• Used only when vehicle is not moving and the engine is running
• Engine may be running for battery charging, cabin or battery temperature control, or catalyst warm-up.

Positive Split Mode

• Engine is ON and driving the generator motor to produce electricity
• Power from the engine is split between the direct path to the road and the path through the generator motor
• Generator power can flow to the battery or to the traction motor
• The traction motor can operate as a motor or a generator to make up the difference between the engine power and the desired power
• This is the preferred mode whenever the battery needs to be charged or when at moderate loads and low vehicle speeds

Negative Split Mode

• The engine is on and the generator motor consumes electrical energy to reduce engine speed
• The traction motor can operate as a motor or a generator to make up the difference between the engine power and the desired power
• Typical highway mode
• Occurs when the engine needs to be on, the system can not be operated in parallel mode and the battery is charged near its upper limit

Electric Mode

• The vehicle is propelled by stored electrical energy only
• The engine is turned off
• The tractive torque supplied from the traction motor
• Preferred mode whenever the desired power is low enough such that it can be produced more efficiently by electrical system than engine
• Preferred mode in reverse because the engine can not deliver reverse torque
• Separate electric pump maintains power assisted steering

 

Electric mode can only be achieved when traveling 63 MPH or less.

 

Negative Split Mode kicks in at speeds higher than 63 MPH and when battery is fully charged.

 

I am now getting EPA mileage - ~47 MPG.  I usually am driving in a mixed scenario - 70% freeway, 30% street

 

On the freeway I often travel faster than 63 MPH (65 - 75 MPH).  I am seeing low to mid 40's average when in Negative Split Mode.  I recommend getting into Negative Split Mode when freeway driving.  Don't try and stay at 63 MPH or less - you won't get the mileage you want.

 

On the road I'll see EPA or better mileage as I can leverage Electric Mode.  I do employ P&G to an extent but not to where I am frustrating drivers behind me.

 

These numbers are only possible in warmer weather - > 45 F.  When temperatures get colder the car will run in Series and Positive Split Modes more often to heat the catalyst or maintain temperature in the cabin.  The hit to MPG dependS on how cold it is.  In single digets to sub zero weather I was getting 35 MPG.  I haven't driven yet in hot weather so not sure how Max AC will impact mileage.

 

This HEV architecture is similar to the Toyota Prius.  Big differences are C-Max has a much larger gas and EV traction motors giving the C-Max much better power and acceleration.  The C-Max isn't as aerodynamic and is heavier than the Prius.  I believe the weight, aerodynamics negatively impact the C-Max's MPG particularly in the cold.  I've heard the Prius gets impacted in the cold but not as much.  C-Max is far more comfortable though with better room and quieter ride.

 

I am very happy with my choice to go with the C-Max over the Prius.

[plus 3 golfer]

For those not aware, this is described in the OBDII Theory and Operation available here.  For anyone that wants to understand the C-Max operation it's a good 154 page read. ;)  Here is the direct link to the pdf.  The propulsion mode described above starts at page 8.

[Bob999]

There are two items in the description that make me wonder about its validity:

 

1.  There is a reference to "parallel mode" (in the description of negative split mode) that is not defined or otherwise discussed; and

2.  The statement that a separate electric pump maintains power assisted steering is, I believe, incorrect.  My understanding is that the CMAX power steering is electric--not an electric motor powering a hydraulic pump to provide hydraulic power for power assisted steering.

[plus 3 golfer]

Series, split, and parallel refers to the path of the power distribution from the engine.  This is my interpretation (although maybe wrong). :) :

 

Series - speed is zero and engine is on.

 

Split - Positive - engine is on, engine power is split between charging battery via the generator and driving the car

Split - Negative - engine is on, engine power is split between the generator consuming power and driving the car

 

Parallel - Generator is stopped (not spinning), engine power and battery assist power (traction motor) driving vehicle. I believe this is what is seen when high speed cruising and mpg is high.  Usually battery would need to be near full charge, otherwise one would likely be in split positive. 

 

Also, I can see how the electric power steering pump statement under the electric mode might imply that a different type of steering is used in other modes. But nowhere does it say a hydraulic system is used.

 

From page 147 of the pdf (note my bolded text):

 

The primary function of the Powersplit transaxle is to manage torque between the electric motors, engine, and driveline. The planetary gear set provides series, parallel and split paths for power distribution from the battery and engine. The torque ratio between the series path and the parallel path is fixed by the geometry of the planetary gear set. The power split between the series path and the parallel path is determined by the relative speeds (all series if vehicle speed is zero and engine is on; all parallel if generator is stopped; split otherwise)

[ScubaDadMiami]

Why does remaining at 63 MPH or less not result in getting good fuel economy?  Others have called 63 to 66 the sweet spot.

 

Nice write up.

[plus 3 golfer]

Best FE is different than most efficient HEV operation.  The HEV should be controlled to maximize HEV efficiency which generally maximizes FE.  The HEV control logic doesn't know how one is going to drive the vehicle.  So, it should run the engine at the most efficient point on the Brake Specific Fuel Consumption curve given the conditions at that time.

 

What I see is that when operating between about 63-66 mph, the FE is significantly better than operating at say 70 mph (more FE difference than caused by the reduced aerodynamic drag due to the speed difference).  So, the sweet spot reference for me is the maximum speed band where the HEV operation is maximized (operating at the engine "sweet spot" on the BSFC to propel the car and to charge the batteries and still allow switch over to EV operation).  This appears to be around 63-66 mph. 

 

I certainly get better FE at 55, 45, and 35 mph speeds than the "sweet spot" speed but that is likely due to significantly reduced aerodynamic drag since the vehicle should still be operating near the BSFC sweet spot and also under EV mode given the same conditions as the sweet spot speed.  Above the sweet spot speed, the engine may have to start operating outside the BSFC sweet spot AND EV mode very rarely occurs if at all.

 

Bottom line: one cannot outwit physics but can take advantage of it.  If time is not a factor, driving below the sweet spot speed will yield better FE all other things being the same.  The only other factor I can think of that might come into play is the operation of the active grille shutters which affects aerodynamic drag.  So, the degree to which the shutters may be open or closed at various speeds around the sweet spot speed could affect FE around those speeds such that under certain conditions one might see lower FE say at 60 mph than 63 mph due to changes in aerodynamic drag caused by shutter operations (speed is one of the variables used to control the shutters). For example, once the car is at operating temperature, more speed will increase air flow through the grille which could reduce coolant temperature and could cause the shutters to close a few notches from an otherwise more open position and thus improve aerodynamics and FE. 

[Recumpence]

I know two people who own Prii. Both have told me they average (in town) 40 to 42mpg in winter and 50mpg in summer. That is what I average (I see about 42mpg in winter and 55 in summer). I find the difference in power, size, and weight to be huge, yet the difference in mileage to be very slight.

 

That, in my mind, make our mighty C-Max far more desireable.    :)

 

Matt

[nsteblay]

My comment on Negative Split Mode relates more to getting the battery fully charged in freeway driving.  I use to drive looking at the gauges trying to maximize my time in EV mode staying at 63 MPH or less. This was a distracting way to drive and didn't result in optimal fuel mileage for me.  Now I stay with the freeway traffic and allow the batteries to fully charge.  I then see the good mileage at higher speeds.  When I have to slow down because of traffic I go into EV mode and apply P&G.  I don't know, maybe its just the warmer weather that is driving my better mileage.  I suppose if I drove at slower speeds I would get even better mileage. The great news is I don't have to and am still getting the EPA mileage.

[SnowStorm]

For those not aware, this is described in the OBDII Theory and Operation available here.  For anyone that wants to understand the C-Max operation it's a good 154 page read. ;)  Here is the direct link to the pdf.  The propulsion mode described above starts at page 8.

+3   Thanks so much for this link.  Looks a lot more interesting than the owners manual.  Now, if someone could just find the BSFC graph...