[chuckles]

Hi, I was wondering if anyone could tell me how energy efficient the honda insight is. I'm not looking for gas mileage, but the percentage of energy that gets produced into forward motion. For example, an average gas powered car is about 30% efficient. I am doing this for a University Environmental Issues assignment. I'd Appreciate any help. Thanks

Chuck

 

[Rick]

30% seems high to me. I was under the impression that ICE engines were more in the realm of ~10% efficient, with most of the other 90% going to heat. I'd imagine with the Insigh's light weight and low friction everything it would make a few percent difference, but I really can't say for sure. Anyone else?

 

[b1shmu63]

I had heard over 40 percent for the hybrids and got this from a dicusion of hydrogen powered cars that was featured on the EVworld site. It was with regard to the theoretical efficiencies of Ice and H2 fuel cells. Check their archives. Note that in the lean burn mode the efficiency of the engine increases dramatically. I had heard years ago of some stationary ICE engines with some really heavy and arcane technology getting 80 percent efficiency, but they would be bigger than most garages! Sorry I can't give you any solid leads.

 

[clett]

I can't help you with outright efficiency of the Insight, but I can give you some pointers for engine efficiency in the Prius. Most petrol engine cars use the Otto cycle, where the compression stroke is the same length as the expansion stroke, and efficiency is therefore limited to ~25-30%. However, a more efficient cycle, the Atkinson cycle, has been mimicked in the Prius by simply closing the inlet valves a little late on compression. This means that the Prius engine effectively has a compression ratio of 9.5:1, but an expansion ratio of 13.5:1. The result is a whopping peak thermal efficiency of 36%. FYI, the highest peak thermal efficiency of any engine on the road is the 1.2litre TDi engine in the Lupo and A2 3L, with peak thermal efficiency of ~45% (diesel engines are inherently more efficient, but this one has been souped up even more).

But, that's not really where hybrids score the most points on efficiency. By having a smaller engine in the Insight, or by using the planetary gear system in the Prius, it means that the engine can run at a wider throttle opening for more of the time (or even full throttle for almost all of the time in the Prius) than a normal car can. Engine efficiency plummets at low throttle openings, and by working the engine harder, you get better efficiency. You can see what I mean in this presentation: http://john1701a.com/prius/presentations/prius_ths-presentation_07.htm Regenerative braking also restores a small fraction of the energy lost from braking in todays mild-hybrids.

In terms of overall vehicle efficiency (not just engine efficiency) there are a million other variables to think about. The gearbox will lose up to 10% of engine power. The size and type of tyres, the aerodynamics and weight of the vehicle all impact efficiency, so there is almost no way of coming up with an "outright" vehicle efficiency for any vehicle.

 

[Rick]

I stand corrected.

 

[Tim Maddux]

there is almost no way of coming up with an "outright" vehicle efficiency for any vehicle.

Sure there is. Divide the net power output by total power input.

The net power output by the wheels against the road has to balance the power consumed by aerodynamic drag against the car. The total power input comes from the gasoline burned. You can estimate reasonable numbers for these, just pick a constant speed (say 50mph) and mpg (say 100mpg) on a flat road. Let the math do the rest.

 

[b1shmu63]

True confession, I once owned a two barrel 400 cubic inch Ford that probably had a thermal efficiency of about 3.5, and unstopable rust! Sold it to Captain Hook for 50 bucks and bought my first Honda, a 1980 Prelude. Sold the Prelude 13 years and 230,000 miles later for 500 dollars to a kid that wanted it for ice (frozen lake) racing.

 

[clett]

Sure there is. Divide the net power output by total power input.

You can do this, but this will only give you a measure of how much chemical energy contained in the fuel is delivered to the driven wheels. It will also be extremely specific to immediate road-conditions: engine and drivetrain efficiency is very different for every throttle opening, every road-speed, every gear, every climate, every altitude etc. Also, this approach would only be able to measure the efficiency of the engine and drivetrain. The vehicle weight, the aerodynamics and all sorts of other things affect "real life" miles-per-gallon, and the effects of these would be missed by measuring output at the driven wheels only.

Theoretically however, there's no real way of defining how 'overall' efficient a car is. If a vehicle drives from sea-level in one place to sea-level a thousand miles away, the stationary car contains just as much potential energy as it did from where it set off - no energy has been gained but lots of energy has been spent! So in the strict physics sense all cars are zero percent efficient!

 

[Tim Maddux]

this will only give you a measure of how much chemical energy contained in the fuel is delivered to the driven wheels.

Right. That is a measure of efficiency, a good enough one for a homework assignment. You could of course make better observations with a dynamometer test.

there's no real way of defining how 'overall' efficient a car is.

Sure there is. I just stated it. Efficiency is defined as the ratio of work done to energy input. Even though the car gains no potential in your sea-level to sea-level trip example, a lot of real work is done in getting it from one place to another. That work is observable. So is the energy input.

 

[clett]

Efficiency is defined as the ratio of work done to energy input. Even though the car gains no potential in your sea-level to sea-level trip example, a lot of real work is done in getting it from one place to another. That work is observable. So is the energy input.

OK, but using this method if you compare a petrol powered Insight with, say, a diesel powered 2 tonne SUV, both might get 30% of the chemical energy of the fuel to the wheels. So by that measure they're equally efficient! Probably why most people just leave it at MPGs!

 

[Guest]

OK, but using this method if you compare a petrol powered Insight with, say, a diesel powered 2 tonne SUV, both might get 30% of the chemical energy of the fuel to the wheels. So by that measure they're equally efficient! Probably why most people just leave it at MPGs!

Yes! The trouble comes when you start equating engine efficiency with miles per gallon. Things like weight and aerodynamics will effect mpg, but NOT efficiency. Efficiency is more or less a constant, while mpg is inversely proportional to amount of work being done.

 

[Tim Maddux]

That's true. Much of what the Insight does is designed to reduce that work done moving the car around (low Cd, low rolling resistance tires). However the original poster was asking for energy efficiency, not fuel efficiency.

I suspect that the Insight would still be ahead in energy efficiency because of steps the Honda designers took to reduce losses in the engine:

http://www.insightcentral.net/encyclope ... ction.html

Also, when operating in lean-burn mode the Insight gets much more complete combustion of the gasoline than a standard ICE. This makes the car more energy efficient, because it dumps less unburned or partially burned fuel out the tailpipe. You can see this in its low levels of CO emissions:

http://www.insightcentral.net/KB/compare/emissions.html

 

[Sheepdog 44]

As a purely academic exercise, i decided to see if i could determine the engine efficiency of Lean Burn in a Honda Insight engine from MetroMPG's real world testing. Well, the Former UFO is not stock, so the one constant i had to approximate was the CD. Stock cd is .25, UFO is under .25cd. So take this approximation with a grain of salt. Also the MPG calculator neglects certain km/h values, since the primary values are in mpg. For example, you can not find mpg at 70km/h!

MetroMPG's real world numbers were compared to a theoretical Honda Insight using the Aero/RR/MPG calculator, which was really informative, since any unknown variables are accurately expressed in the real world mpg he got.

Raw numbers: (All values are for Lean burn in 5th gear)
Speed ........... AVG ..........

km/h . mph . MPG (US) ENG EFF% .....%MPG Gain vs non LB

50 ... 31.1 ..... 130.9 ...... 18.7% ................00%
60 ... 37.3 ..... 121.5....... 22.4% ................12%
70 ... 43.5 ..... 113.9 ...... 24.7% ................24%
80 ... 49.7 ..... 98.0 ........ 25.2% ................27%
90 ... 55.9 ..... 88.5 ........ 26.5% ................34%
100 .. 62.1 .... 81.3 ........ 28.1% ................42%
110 .. 68.4 .... 67.1 ........ 25.9% ................39%


You can see engine efficiency goes up with speed, as the engine experiences higher loads. Those are pretty good numbers for an engine experiencing low load on a steady flat road. Lean Burn nearly approaches the engine efficiency of high load in a pulse and glide regimen. Which would be the condition where an engine experiences peak efficiency. And corroborates the story that Lean Burn gives more or less the same mpg as Pulse and Glide at certain speeds.

 

[retepsnikrep]

Interesting that although the engine might be working efficiently at 100kmh the mpg is much lower than 30mph as one would expect. Thanks for that.

 

[neloner]

You need to ask at what speed you are talking about as well, as aerodynamics will play a role in energy efficiency at a different rate depending on the speed being travelled.