The 30 mph challenge

[bwilson4web]

Originally Posted by Deezler

Who the heck drives steady state 30mph anyway?

The average speed of the EPA City cycle is 28 mph so 30 mph happens to be very close.

Originally Posted by Deezler

I really don't understand how engine friction at 30 mph is a "problem".

Because of the low vehicle power demands, ~5 hp, it can be treated as a constant for engines of similar displacement. So we don't really have to bother with "a function fo speed (rpm) primarily and load second." A steady speed eliminates another red herring.

Originally Posted by Deezler

Saying 50% at 30mph is a very poor blanket statement.

Actually it is fairly accurate in the case of vehicles that must run their engine to maintain a constant speed. As pointed out, the 5 hp starting motor power gives a good approximation of the engine, internal power loss at low power settings. To provide an equal amount of motive power, 30 mph requires pretty close to the extra 5 hp needed.

Originally Posted by Deezler

I would be interested to compare the FE of a prius steady at 30mph vs. a diesel VW allowed to run P & G in the 25-35 range as well.....

Actually I would like to see steady state, MPG vs mph for a diesel similar to what I already have for the Prius:


In the meanwhile, the EPA City ratings tell the tale when identical driving profiles are used:

• 29 MPG - Jetta diesel automatic
• 51 MPG - Prius 2010 automatic

But since P&G seems to posses cache (regardless of the traffic issues that limit its application,) I would be just as happy to see the Prius vs Diesel P&G contest. The reason is we have already run the NHW20 Prius at P&G averaging close to 30 mph:


Seriously, do diesel advocates really think they'll get in excess of 100-115 MPG as the Prius team did in August 2005?

Bob Wilson

[Deezler]

Originally Posted by bwilson4web

The average speed of the EPA City cycle is 28 mph so 30 mph happens to be very close.

Exactly, I said steady state, and you are talking about average speed. Even the city drive cycle consists of multiple periods of acceleration followed by cruising at speeds a good bit higher than 30 mph. Furthermore I still think that driving a car in an ultra urban environment is a pretty stupid way to get around.

Originally Posted by bwilson4web

Because of the low vehicle power demands, ~5 hp, it can be treated as a constant for engines of similar displacement. So we don't really have to bother with "a function fo speed (rpm) primarily and load second." A steady speed eliminates another red herring.

Playing with the aerodynamic calculator on this site just now one can find required road load power levels at 30 mph of anywhere from ~2.5 HP for a small car to over 10 HP for a truck or SUV. But I was referring to engine speed (rpm) here, not vehicle speed.

Originally Posted by bwilson4web

Actually it is fairly accurate in the case of vehicles that must run their engine to maintain a constant speed. As pointed out, the 5 hp starting motor power gives a good approximation of the engine, internal power loss at low power settings. To provide an equal amount of motive power, 30 mph requires pretty close to the extra 5 hp needed.

Again you are lumping all IC engines into one number. I measure internal engine friction at work (auto industry supplier dyno lab). A large V8 engine we just ran consumes 17 HP just to spin at 2000 rpm. This engine of course has the low end torque capability to drive most cars at less than 2000 rpm, so its FE performance is going to depend entirely upon vehicle gearing. A small 4 cyl engine will often show less than half that much power loss.
So your average engine / vehicle approximations are crude at best. The balance point of vehicle power requirements to engine friction power consumption will vary.

Originally Posted by bwilson4web

Seriously, do diesel advocates really think they'll get in excess of 100-115 MPG as the Prius team did in August 2005?

Probably not (although 80+ mpg wouldn't surprise me). I hadn't seen your data, and it is impressive. How long a period of time do you obtain your data for? I would imagine once the battery is drained your mileage plummets. Hence the advantage gained in city driving with regen braking. But long stretches of road where one can drive steady state at 30 mph don't really exist, so I still find the whole conversation kind of moot. For the same reason I'm not going to offer to test my own vehicle. I bought it for highway commuting.
Oh and please refer to AMCI obtaining 38 mpg city for the new VW diesel. EPA is a pile of Sh*t.

[tjts1]

Originally Posted by bwilson4web

Because of the low vehicle power demands, ~5 hp, it can be treated as a constant for engines of similar displacement.

You really have no clue what the hell you're talking about. This is a random number you pulled out of your ass and you keep repeating over and over again in the hopes that it will stick as 'fact' in people's minds. This is pure fantasy.

[tasdrouille]

Apples and oranges.

The Prius was designed and marketed to be the ultimate fuel economy production car, which it is.

The TDI is just an engine option in a brand of cars designed around driving passion, but it just happens to have pretty good fuel economy too! There was no exceptional attention paid to RR or aero with the TDI.

Put a diesel ICE in a Prius-like car and the diesel wins hands down.

BTW, according to Wayne, who's got to try pretty much all hybrids and the Jetta TDI, over 60 mph the TDI starts to pull away.

[NiHaoMike]

About how much power does it actually take to maintain a speed of 30MPH in a Prius on level ground with no wind? I'm thinking it might be possible to install a bicycle generator in front of the passenger seat and use that provide a fair share of the power needed. (And of course, I really like the idea of Lauren Stanhouse acting as an APU for the Prius. Enable EV mode, let Lauren pedal to keep the battery charged, and at some impractically low speed, it can maintain an "infinite" MPG for a very long time. Add some more bicycle generators and fat chicks and it might actually be able to maintain a constant 30MPH on a track...)