no electric for pulse and glide?

Discussion in 'Gen 2 Prius Main Forum' started by mdh, May 25, 2006.

  1. mdh

    mdh New Member

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  2. daniel

    daniel Cat Lovers Against the Bomb

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    Yes, it's true. The gas engine in the Prius is extremely efficient, but has insufficient torque for most peoples' idea of how a car should handle. Combining the electric motors into an integrated system (HSD) allows the Prius to perform like a "normal" car with its much more efficient engine. But if you are willing to live with the extreme limitations of P&G driving you avoid most of the conversion losses of charging and discharging the battery.

    Note that a conventional car of the same weight with an extremely small, efficient, low-torque engine would also get hyper mileage. But it would not be safe for driving in traffic due to its inability to accelerate at an acceptable rate.

    Note also that achieving that "no-arrows" condition on acceleration is not easy. It's much easier on the Glide (deceleration) portion of P&G.

    And note that there's always some current leaving the battery when the engine is not running, and over the long haul that must be replaced during the engine-on phase. So you cannot always be in a no-arrows condition.

    If you attempt Pulse-and-Glide driving, be careful to do it safely, with an awareness of traffic conditions, and knowing that other drivers will not understand what you are doing and will not be able to anticipate your behavior to maintain a safe distance from you. They might even think that you are drunk or crazy.
     
  3. Redblue88

    Redblue88 New Member

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    <div class='quotetop'>QUOTE(daniel @ May 26 2006, 10:13 AM) [snapback]261287[/snapback]</div>
    You mean the Prius isn't a perpetual motion machine? I thought I WAS crazy :D
     
  4. mdarby

    mdarby Junior Member

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    Why is pulse and glide more efficient than driving at a steady state if driving on flat terrain? Does "pulsing" put the engine in a more efficient operating range than cruising? Does "gliding" avoid some baseline friction which the engine provides at crusing?

    Thanks!

    <div class='quotetop'>QUOTE(daniel @ May 26 2006, 09:13 AM) [snapback]261287[/snapback]</div>
     
  5. donee

    donee New Member

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    Hi Mdarby,

    Yes, higher engine power levels are more efficient.

    Gasoline engines burn fuel stoiciometrically (proper amount of fuel for the oxygen in the charge) and use the momentum of the exiting burned charge to pull a new charge in. This requires that the intake valve open a little before the exhaust valve closes. This is called "valve overlap". At this time the exhaust gases are rapidly flowing out the exhaust valve. The momentum of the gas forms a vacuum in the cylinder, which is filled by the intake charge. The Puegot brothers invented this, and came over from France and beat the pants off the locals at the Indy 500 to prove it worked. Ever since then automobile engines have had valve overlap. This is more efficient, because just the energy that would be wasted in the exhaust is doing work. Prior to the Puegot brothers, cars reved not much much over 1 or 2 thousand RPM. Afterwards, the state of the art is about 18 K RPM in a F1 race car.

    To control the charge energy (number of molecules of air and gas in the cylinder) during the burn gasoline cars have a throttle to cause a partial vacuum at the intakde valve. This vacuum takes energy to overcome. If more air was fed in than the amount of fuel could burn, the mixture would explode (called detonation in car talk).

    Additionally, due to the aerodynamical action of valve overlap, there are optimum RPM's and engine loads for the best cylinder scavenging, and charging action. I am not sure of the numbers but for standard engines, I believe best efficiency occurs at .6 rated horsepower at about .8 max rpm. Car enthusiasts feel this little extra push and call it "coming on cam". The cam is the device in the engine that sets valve overlap (unless the engine is a VVT). This is where the engine will generate the best energy out for the fuel consumed ratio.

    Now, holding almost any regular car at .6 rated horsepower for very long, and you will be doing 95 mph. While the engine is at its most efficient, the car system is not. Because the car aerodynmic drag force goes up by the square of speed, and power required by the cube of speed.

    The reason standard cars have the power they do is for reasonable accelleration. If the car only had enough power for the typical max speed, say 85 mph, then it would take 3 or 4 times as long to accellerate. Examples of vehicles setup for best typical speed efficiency are trains and 18 wheel truck. Those vehicles have enough power to maintain maximum speed, but dramatic gearing is required for any accelleration at all.

    The problem is that a car at the typical average speed (50 mph - 35 for local roads and 65 for highway) the engine is operating well below the best efficiency point. Standard car engine efficiency can be half (about 12 percent) of what it would be at optimum power/rpm (about 25 percent). Yikes!! Engineers call this the "Partial Power Problem".

    This is actually the technical foundation of the Hybrid concept. By sizing the engine closer to the average power requirement, and using the battery/motor for the power peaks, better acceleration efficiency is obtained as the engine is running closer to its peak efficiency point. Additionally after accelleration the engine can run at a higher efficiency point while only a small amount of power is needed to push the car, as the batteries need to be recharged. And lastly, regeneration can be used for braking the car, and that recovers some of the energy put out by the engine to get the car to speed.

    As an aside, in standard cars its more difficult to use an efficient level of accelleration due to fuel system "enrichment". Both carburator and fuel injection cars will push a little extra squirt of gas on acceleration. This is called "enrichment". Its done to avoid detonation, I believe. This is why people are told to accelerate slowly for best milleage in standard cars, too. Actually, its the rate of accelleration, or how quickly you push the pedal, not what the pedal ends up at that is important for efficiency. This is probably partially responsible for the Prius little old lady - poor MPG phenomena.

    The Prius relies on the battery for rapid start of accelleration function, so as I understand it, they left enrichment out of the engine fuel system. The battery comes up quickly, but the engine comes on load more gradually than a standard car.

    Additionally, the Prius engine has a special way to improve engine partial power efficiency and best efficiency. But this post is already long! At about 1/3 power the Prius engine is already near its maximun (30%) efficiency. So, the Pulse needs to get the engine power up to about 1/3 to 1/2 the engine power until one reaches a speed where the power to overcome aerodynamic drag becomes wasteful. Remember, that is a very sharp function (cubic). During the pulse the CVT does the speeding up, while the engine comes up to a high efficiency point.

    At the average of the pulse/glide (35 mph), the power required is well below the 1/3 engine power even for the smaller Prius Engine (76/3 = 25.3 hp) . The 28 hp battery can even push the car along (not for long) at 55 mph, including cranking the unfueled engine over. At 35 mph, power needed is probably under 10 hp.
     
  6. Vincent

    Vincent Don't Wait Until Tomorrow

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    Wow!! Great Pulse and Glide Response.
     
  7. haceaton

    haceaton New Member

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    <div class='quotetop'>QUOTE(donee @ May 30 2006, 09:38 PM) [snapback]263129[/snapback]</div>
    All true, but the real advantage of the pulse and glide comes from the fact that during the glide phase, the engine is stopped (i.e. it's not spinning). This completely elliminates pumping losses and a great deal of friction. The electric motors are both spinning, but are consuming no power in a true glide. It is quite a bit like coasting in a traditional manual shift car in neutral or with the clutch in. Of course in those cars there is no convenient way to turn off the engine, nor restart it.

    The genius of the Prius is that it provides a safe, reliable and controlled way to cycle the engine at low speed. The reason that the upper limit for pulse and glide is 40 MPH is because above that speed the engine must spin, resulting in significantly more friction. Of course the aerodynamic drag goes up too so that's another reason to keep the speed low.

    I just got my new Prius yesterday: 2006 Barcelona Red Package #8. It had 5 miles on the odometer when I got it and the consumption display showed 5 miles and 16 MPG average!!!! After putting on another 50 miles I've got the average up to 40 (thus I averaged 47, not bad for rush-hour traffic), but it will be a challenge to get a really good number by the end of the tank. The new Prius energy display updates much faster and doesn't seem to be seriously delayed as in the classic Prius (where the energy display is effectively useless); You can actually use it to try to learn to glide effectively. I will say that the slightest variation in pedal position in either direction takes it out of glide so it will take me some time to really get the hang of it.

    I was surprised by how often there were good opportunities for P&G in a normal rush-hour, non-highway driving situation. Often times I had to glide down below 30 MPH because the traffic ahead wouldn't allow speeding up.
     
  8. daniel

    daniel Cat Lovers Against the Bomb

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    <div class='quotetop'>QUOTE(Redblue88 @ May 26 2006, 07:24 AM) [snapback]261291[/snapback]</div>
    I know you're joking. But since the engine runs during the pulse phase, nobody is suggesting you get something for nothing. They're just claiming you get a bit more for a bit less.

    And I was pointing out that since there is always some battery drain when the engine is off, it's impossible to entirely avoid arrows to/from the battery, as that drain requires recharging from the engine later. (Or recharging from re-gen braking, but since the glide uses no re-gen, it's got to recharge from the engine.)

    donee: Thanks for that explanation. Please feel free to make your posts as long as you like. Substantive posts like that one do not get "too long."
     
  9. joelparks

    joelparks New Member

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    <div class='quotetop'>QUOTE(donee @ May 30 2006, 09:38 PM) [snapback]263129[/snapback]</div>
    Beautiful explanation, donee. THis ought to be in the Knowledge Base, eh?

    The only thing missing is the logical conclusion that haceaton adds, namely that since the average power needed to maintain an average of 35mph is less than 1/3 the rated ICE power, then because the losses of starting and stopping the ICE are small (although still present*) it is most efficient to "pulse" up to nearly 42mph and then coast with the ICE off and no (minimal) power draw from the battery until the speed drops to <as low as you and surrounding traffic can tolerate>.

    * The losses from starting and stopping the ICE are small but not negligible. In fact when the battery gets overcharged, the HSD will spin up the ICE (without combustion) and then stop it, over and over in order to bleed off the excess charge. So to minimize this non-negligible loss, it is best to stretch out the P&G cycle to minimize the total number of times the ICE is started up on any given trip.

    Which leads me to my question: using my MFD (or my CAN-View if necessary), what is the best way to tell when I have reached the point on the ICE power curve where the efficiency is high? I live in an area of many small hills, so accelerating away from an intersection is as likely to be uphill as downhill or level, so I don't think I can rely on trying to maintain a given instantaneous fuel consumption value... or can I? The reason I ask is so I can know how slowly I can accelerate while still hitting the power band on the ICE rpm/power curve
     
  10. darelldd

    darelldd Prius is our Gas Guzzler

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    Donee -

    That was excellent. I'm going to swipe it and find some place to put it on my site for sure!

    I'd like to add (just because I'm annoying that way) that Electric Vehicles have very little (close enough to call it "none" I think) of these crazy efficiency differences at different engine loads. After a certain low speed to overcome the fixed frictions, an EV gives you exactly what you need when you need it. No more, no less... and VERY little loss. The hybrids will only get better as the gas engines get smaller and the electric motors (without having to get bigger!) take on more of the traction tasks. If we can get that ICE to spin at mostly a steady speed, and soak up all the bumps with the motor, we'll be in pretty good shape!
     
  11. ken1784

    ken1784 SuperMID designer

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    <div class='quotetop'>QUOTE(joelparks @ Jun 13 2006, 01:23 PM) [snapback]270364[/snapback]</div>
    Our Japanese mileage enthusiasts found the best accelerator pedal value (CANID 0x244 #6) is 75 to 80 on the flat road.
    On the CAN-View, it is 37.5% to 40% pedal value.

    [email protected]
     
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