Easy way to explain why Prius gets higher City MPG than highway

I would answer that it doesn't get better mpg in the city. It is simply the not real world driving test of the EPA. Around town I'll get around 44 mpg and on the highway doing 65 mph I'll get 48-50 mpg. The best mpg is on country roads doing 55 mph. The only way to get better mpg in the city is to have an EBH and do P&G. It doesn't get better city mpg b/c of the acceleration from dead stops.

 

I get around 50mpg highway and 60 in the city - which I guess is pretty close to the EPA figures, and it makes sense it would work out that way. At higher speed wind resistance is higher and at constant speed the hybrid system doesn't get much chance to work, nor can one glide (due to wind resistance rapidly causing speed loss).

In the city, slower speed means lower wind resistance, regerative braking at intersections and in traffic flow in general reclaims energy that would otherwise be lost and allows it to be reused instead of supplanted by more petrol, and lower mean speeds allow the engine to operate more efficiently.

Not sure that would make much of an explanation for someone not familiar with the car though.

 

I'd probably just explain that his car burns gas when he is coasting, slowing down, and stopped. Mine shuts the engine off at speeds less than 40 mph when I coast or slow down, and also when I'm stopped. This allows my car to avoid wasting a lot of fuel in the city when the engine isn't needed.

There is a lot more to it than that, but that is usually enough to satisfy those who are just mildly curious.

 

With the stop and go in the city due to red lights I get horrible gas mileage and way better on the freeway. Bottom line is that the electric motor doesn't have enough energy to move the car from a stopped position unless you accelerate VERY VERY slowly (and piss everyone off behind you in the process).

Stop and go traffic is another story. I get amazing gas mileage in traffic because I'm idling most of the time and only using the battery to move it up to 5 mph.

Quick update: I average about 20-30 mpg in the city.

 

To me, the simple explanation is city mileage is at a speed of, say, 20 MPH on average (only when moving). The Prius design allows us to remove most of the fuel lost while idling and most of the fuel lost accelerating and slowing because of regen and engine shutdown when stopped.

On the highway, the higher air drag lowers mileage - so you are comparing mileage at 20 MPH vs 60 MPH though there are losses at the lower speed due to conversion inefficiency.

Note that to get -best- mileage the Prius (and most other cars I've measured) should travel at 45-50 MPH. Prius will get phenomenal mileage at those speeds - I see 64 MPG or so (3.7 l/100 km). If only it were safe to cruise at those speeds.

 

<div class='quotetop'>QUOTE(Bendial @ Aug 1 2007, 01:53 PM) [snapback]488846[/snapback]</div>

It's the driver, not the car.

 

<div class='quotetop'>QUOTE(usbseawolf2000 @ Aug 1 2007, 01:28 PM) [snapback]488821[/snapback]</div>

A vehicle traveling at a constant speed uses less energy, however.

 

Hi All,

The EPA "City" Cycle is more similar to suburban driving with stops well spaced (1 mile or so). This is not like driving downtown in a large city during rush hour. So, its not really a "City" cycle, but an average urban/suburban cycle.

Most of us here probably agree that the Prius really does well in an average urban cycle. With some slow (35 to 55 mph) highway driving, and secondary road travel (40 mph speed limit). 50 + 25 C (epa average test temp) summer mpg is easy to obtain, even in 90 degee weather. I am at 59 mpg at the moment.

Now how to explain why this is to the average, knows-enough-to-be-dangerous, automotive ludite? Even my dad, who is a PHD ME, with AF training in engines, and knows all about big gas turbine vibration stuff, took a few moments to realize this when I explained it.

Yes, while Cars typically have bad average urban cycle fuel economy, there is really no basic physics reason why. At slower speeds, cars have less air drag. So, the engine only really needs to overcome hill climbing, accelleration and rolling resistance. Once at speed, on level terrain, the power required is so little, one can coast a long way with the enige off. At highway speed however, air drag is very large. So really, the question is why do cars get better highway mileage, if they need to expend so much power to just keep moving?

There are three primary reasons the Prius has good FE in the average urban cycle, when other cars do not. The first I pointed out to my dad is that "Part Throttle" problem. Which of course he knew all about. So, you have to explain that in simple terms to the guy off the street who does not have the technical background, or whom you do not he has the background. An Otto cycle engine has its best efficiency - output energy delivered per fuel energy consumed , about 5/8 ths power at heavier torques. This is because of the internal aerodynamics of the piston engine, too. At this setting the throttle plate is close to wide open and offers little air drag. But its also because of the cam profile, and valve overlap (thank-you Puegot brothers). Due to air-flow pulsing, energy that would normally be wasted in heating the air in the intake and exhaust manifolds, helps to squeeze the air through the valves more efficiently.

At lower RPM's and airmass flows, the Otto engine is less efficient. Typically 1/3 to 1/2 its best efficiency at 1/4 th power - the typical cruising power in the average urban cyle. Or about 10 % efficient instead of 20 to 25 % efficient. Think about that. Even at its best 3/4 of the gasoline energy is wasted in a standard car engine. So, if there was someway to use just a little bit more of that gasoline energy, great improvements in fuel economy would result. And even more, if there was a way to use the Otto engine at its best fuel economy only - that alone would double the city mileage! In a standard car we just can't turn the engine off and coast down the road (legally, or unless you are Wayne Gedes!). And what about the AC, Power Steering, and Power Brakes? These all need at least an idling engine to work properly.

This is part of why highway fuel economy is better in traditional cars. The engine 1 1/2 to 2 times the efficiency at the higher highway level of engine power, the car does not need that much more gas. The full efficiency is not realized even at highway power level however.

Additionally, to get rapid engine response, a standard car has what is called "enrichment". When you depress the throttle quickly, an extra squirt of gas is sent to the engine to make it rotate quicker , more suddenly. This is the source of the "accelerate gently for best gas mileage" recommendation. This extra squirt of gas happens each time you start from a stop, or have to accelerate quickly in traffic. It does not happen when one is casually cruising down the highway at constant speed.


Enter the Hybrid concept. What if a battery and electric motor could take over at low loads, and during initial accelleration? Then the engine then can come up to speed slowly, while the car jumps out from a stop quickly. The engine would only run when the car required high power. Additionally, charging the battery when the car is cruising, increases the load on the engine, and the efficiency it runs at. When the battery is charged, the engine can stop, and the car continue on the battery.


Second, is the Prius does not use an Otto cycle engine. It uses an Atkinson cycle engine, which does not have as high a power as the Otto Cycle, but has good efficiency over a wide load band. Argonne Labs tested the Prius engine, and found it still had 25 % efficiency at 1/6 th its max HP - as good as a the best Otto cycle efficieny! A HP level that is just about right for cruising and charging the battery at 40 or so mph. At its best efficiency, it was well better than an Otto engine - thus good highway mileage too.

And the third reason the Prius has better average urban cycle fuel economy than other cars is the side benefit of the electric hybridization - regeneration. In slow and go traffic, a slowing car has to brake sometimes, or downhill. In the Prius, that energy is recovered and can be reused at about a 40 % efficiency.

 

Bendial -- idling? 30 mpg? Do you have it in "B" all the time?
.
_H*

 

I think his emergency brake is on

 

20-30? Ouch.

I've only had it a few hundred miles but I'm getting about 54 highway and when I'm doing city driving it nudges up from there.

I do almost all highway, but the fact that I'm beating the EPA estimate while doing 70MPH on a regular basis makes me all happy.

I've never done P&G or anything like that - the only change to my driving technique is that I try to brake less and do not do jackrabbit starts. I just drive "smoother".

 

<div class='quotetop'>QUOTE(usbseawolf2000 @ Aug 1 2007, 04:28 PM) [snapback]488821[/snapback]</div>

In stop-and-go driving you're driving at slower speeds, which is more efficient, combined with being able to take advantage of regen, which is minimized during highway driving.

My daily commute is split between freeway and stop and go back roads. I average around 52 on the highway and 56-58 in stop and go (MFD computer).

 

donee
"The EPA "City" Cycle is more similar to suburban driving with stops well spaced (1 mile or so). This is not like driving downtown in a large city during rush hour. So, its not really a "City" cycle, but an average urban/suburban cycle."

I guess this would explain why I get around 40mpg tops around my town with stop signs and lights spaced about 1/4 mile apart. Most towns on Long Island are designed in this manner. I have to make at least 10 to 15 full stops before I can hit a highway. Driving around town on a normal day I cannot drive more than 2 minutes before I have to stop (even with anticipating stops and pulse and glide).

All those who are getting 50mpg+ city mileage must live in your ideal suburb.

I am still extremely happy with my 40mpg/city 55mpg/highway over the 8mpg/city and 12mpg/highway I was getting on the vehicle I replaced.

 

<div class='quotetop'>QUOTE(ZA_Andy @ Aug 1 2007, 03:43 PM) [snapback]488835[/snapback]</div>

I actually find I get better milage on the highway. I usually get 51-52 MPG consistently on a fairly long highway trip. In the city, I tend to average around 46-49 MPG. That is mostly due to lots of stop lights which cannot be avoided.

 

It's possible to get high MPG in a place with traffic lights every city block. But then you must be prepared to spend much time around 15 or 20 mph. This of course tends to annoy people behind you, so it's an engineering question: what is worth more to you?

The city-driving component of my commute to work involves traffic lights that are separated by half miles. I do loads of gliding between them. I also follow the watch-three-lights-ahead rule, and while it's no panacea (I am not very good at anticipating yellow lights), it helps. As a result my MFD tends to show bars around 55-60 for the highway and then bars of 80+ for the city streets.

 

I do alot of the coasting between lights if the upcoming one is red. However, I tend to accelerate normally from stoplights especially if there are people behind me since I know I would not want someone slowly accelerting from a stop light which stunts the flow of traffic.

 

<div class='quotetop'>QUOTE(Washington1788 @ Aug 2 2007, 10:55 AM) [snapback]489395[/snapback]</div>

I sometimes get someone behind me who get's aggravated, zooming up and stopping, thinking that they are going to go faster through town because they're in a hurry. Since I happen to know the light timing, I usually pass them at the next light they just zoomed up to and stopped, as I'm already rolling at the in town limit and they are accelerating from a stop. It usually takes 2-3 lights before the light bulb goes on, they get behind me, and we all cruise through the timed lights at the possible speed...