Last post on Jun 12, 2013 at 9:52 AM
You are in the Honda Insight
What is this discussion about?
Honda Insight, Fuel Efficiency (MPG), Hatchback
#102 of 164 To spunjurno: driving on electric motor only
Aug 06, 2011 (5:09 am)
A while back you suggested that to optimize mileage, you should try to drive on the electric motor only for a long as possible. You described this as "free mileage". I would have to disagree with that recommendation and here's why...
The purpose of a gas/electric hybrid design is to reap the advantages of two different power sources, an internal combustion engine and an electric motor. Small internal combustion engines are great for reducing fuel consumption. They don't produce much torque but a car does not need much torque to maintain a constant velocity (exceptions excluded). Where a car needs the most torque is when accelerating from a stop. This is where the electric motor comes in. Electric motors can be designed to have maximum torque at 0 RPM.
In a hybrid, the electric motor supplements the small ICE (internal combustion engine), providing the high torque that the ICE cannot, when accelerating. That's the principal behind the electric motor/ICE combination.
Now when you say "free mileage", thats not really true. In this hybrid, all of the energy comes from gasoline. (if the Insight was a plugin hybrid, this would be different) Just because you are using the electric motor only to maintain your speed, does not get you anything for free, as a matter of fact, it is the wrong thing to do from an efficiency perspective.
Why is this the case? The ICE generates all of the electrical energy in the battery. Charging the battery is suplemented by regenerative braking as well, but the benefit of this is highly dependent on driving habits and conditions. For the sake of this argument, lets just consider it all coming from the ICE.
Remember, the purpose of the electric motor is to aid acceleration. Depleting the battery to maintain velocity (no acceleration) is less efficient because the energy from the gasoline has to be first converted into electrical energy and be stored in the battery. The electric motor then takes the electrical energy in the battery and converts it to mechanical energy to move the car. This indirect path from gasoline to mechanical energy is less efficient than using the ICE to take the energy from gasoline and convert directly into mechanical energy because there are losses inccured at each level of the process.
However, having an electric motor to generate high torque levels to get the car moving allows the use of a much smaller ICE, which at low torque levels is very efficient at converting gasoline into mechanical energy.
Suffice it to say that using the electric motor to "maintain" your speed is a waste. The battery should be reserved for acceleration because the ICE cannot do this job very well or very efficiently.
You mentioned that you have driven a Prius. On the earlier models of the Prius, i.e. 2006-2009, if you take the Prius on a flat stretch of highway, set the cruise control to 65 or so,... as you approach small rises in the terrain, you can watch as the Prius HSD uses the electric motor to make small contributions of torque to maintain the selected cruise speed, while noticing that the Prius' ICE maintains very much a constant RPM. This is because the ICE run at a constant RPM is being optimized by the engine control computer. Changing the ICE RPM to accelerate reduces the efficiency of the ICE. This finely tuned balancing process in the Prius is very interesting to watch.
Also, you have made some comments about coasting. In the Insight, whenever your foot is off the gas, the engine control computer actually shuts off the fuel injectors to the engine so you are burning no fuel. The ICE continues to turn because with Honda's IMA system, the ICE is always connected to the wheels when the transmission is engaged. What happens is the fuel is cutoff and the valves of the engine are shut to reduce pumping losses. This is a less sophisticated system when compared to the Prius, yet highly effective at a much lower cost.
In light of this, what I have found to reduce fuel consumption the most is to look ahead for any reason you might have to brake. If you see that the light is red ahead, take your foot off the gas and coast. The better you get at this the higher your MPG rating. Also, when you take your foot off the gas, you will notice that the charge/assist meter on the right will show a slight charge level. This is because the hybrid control system is interpreting your action as that of intentionally wanting to slow down. It takes advantage of this by tapping some of the kinetic energy of the car's motion to recharge the hybrid battery. You can take advantage of this feature as well. Let's say you see that the light ahead is red. You take your foot off the gas and coast. The hybrid control system takes an opportunity to slowly charge the battery. The car will slow down a bit because the electric motor is now being used as a generator. Next, when you are getting closer to the light, or the back of another car, lightly touch the brake pedal. A light touch on the pedal will tell the hybrid control system that you wish to deaccelerate at a higher rate. It will follow you by increasing the charge rate to the battery, slowing you down faster. This is regenerative braking. Note that with a light touch on the brake pedal, you are not yet activating the hydraulic disc brakes which simply convert the kinetic energy of the moving car into heat which is wasteful. At the last stage, depress the brake pedal firmly and the hydraulic brakes will bring the car to a complete stop. By doing this, you recover as much electrical energy as you can. This is energy that you will later not have to get from the gasoline.
Oh, and one more thing spunjurno, your headlights don't run off of the hybrid battery (100.8 Volts). Different circuit entirely. The hybrid battery is only used to run the traction motor. If you ran a battery down by having your lights on in traffic, it is the 12V accessory battery under the hood that was run down. The hybrid control computer on the Insight would never allow the situation you described.
#103 of 164 Re: Tips [jbrunelle1]
Aug 06, 2011 (8:47 am)
jburnelle, how about the Volt? It is expensive, but if you can afford it, it has great range and is mostly an EV with a fuel recharger.
#104 of 164 Is the Insight right for me?
Aug 09, 2011 (1:30 pm)
I have a 60 mile commute one way. I live in the Catskill Mountains. I go down the moutain for about 10 miles, then it flattens out for 20 miles and I go 64 MPH on cruise control. Then I get on the Thruway for another 30 miles. Flat and I go 73 MPH, on cruise control. The winter's are long here. And then I have to go up the 10 miles on the way home. Parts of the climb are fairly steep. In my Camry, it jumps to 2200 at about 45 - 50 MPH (it's an older car). I've been reading this terrific forum because I really want an Insight...I like the looks and it's a Honda...but I'm wondering if my commute is wrong for this car? If I go any slower, I would never get to work. I will say, in the winter I go slower because it's dark and I don't want to hit a deer. Any insights (pun intended) would be very helpful. Thanks.
#105 of 164 Re: Is the Insight right for me? [eastkill]
Aug 09, 2011 (6:55 pm)
You will get anywhere from 46 to 50 mpg, even running the air conditioning at that speed, that is supposing that you get a lot of efficiency on the way down and then it goes down on the way back, when you are climbing. You should get 100+mpg on the way down, even at 70, when flat you will get anywhere from 45-50 at that speed and then on the way up it can go anywhere between 30 and 45, depending on how steep the climb is. I have even done 80 (on a 70 road, somewhat flat), with no less than 40-45 mpg. Road conditions are important, but how many mpg's do you get in your Camry?
#106 of 164 Re: To spunjurno: driving on electric motor only [kidsid]
Aug 12, 2011 (10:40 am)
'A while back you suggested that to optimize mileage, you should try to drive on the electric motor only for a long as possible. You described this as "free mileage". I would have to disagree with that recommendation and here's why... "
The Honda hybrid system does not have a pure electric mode. When it is moving, the engine is running.
#107 of 164 Re: To spunjurno: driving on electric motor only [stevedebi]
Aug 12, 2011 (9:57 pm)
Not true. I'm an electrical engineer who put himself thru college as an automechanic. 20+ years of working on cars and designing computer chips. I also own the vehicle,... 2010 Insight EX with Nav, Atomic Blue. I assure you it does.
You can get the electric motor to power the car by itself. If you own one, toggle over to the information display page that shows the battery level. As you are driving you can see how the energy flows to and from the car, battery and gas tank. When you see an arrow pointing from the battery to the car by itself, that means that the electric motor is keeping the car moving. See this youtube link of someone doing it:
When this happens, the gas engine is still turning but it is not running because the hybrid control computer has shutoff the fuel to the fuel injectors.
#109 of 164 Re: To spunjurno: driving on electric motor only [kidsid]
Aug 16, 2011 (6:20 pm)
"If you own one, toggle over to the information display page that shows the battery level. As you are driving you can see how the energy flows to and from the car, battery and gas tank. When you see an arrow pointing from the battery to the car by itself, that means that the electric motor is keeping the car moving. See this youtube link of someone doing it: "
Possibly I should have said that the engine is turning over whenever the car is moving. Strange in a way, because that means the electric motor is having to push the cylinders, which takes more energy. But I suppose that is because the IMA has the electric motors around the driveshaft, making it impossible to completely disconnect the electric from the mechanical (unlike the Toyota and Ford hybrid systems).
#110 of 164 Re: To spunjurno: driving on electric motor only [stevedebi]
Aug 16, 2011 (7:42 pm)
The electrical motor is not having to push the cylinders because the valves are closed, which makes it a push-compress cycle, which does not add to the energy spending. You do have the friction of the cylinders moving though, so it probably is a less efficient system, however, the lower weight should make up for it, at least some.
#111 of 164 Re: To spunjurno: driving on electric motor only [albalma]
Aug 17, 2011 (2:41 am)
Not quite. As you mentioned, there are energy losses do to friction. However, the push-compress cycle you mention is not an adiabatic process. When air is compressed, it heats up. Some of this heat is loss thru the cylinder walls via the cooling system. This heat loss is energy loss that adds to energy spending. Obviously not an ideal situation. A lower cost solution compared to Prius, but still quite effective.