The 30 mph challenge
 

Hi,

This article got me thinking about the 30 mph problem:

Power can co-exist with fuel efficiency

The 30 mph problem is it takes about 5 hp (3.7 kW) to sustain a modern sedan. But this is about the same amount of energy needed by the engine alone to turn over at low power settings. So about 10 hp is needed of which about 50% is used to turn over the engine and 50% used to keep the vehicle moving.

In contrast, a Ford or Toyota hybrid will run the engine for a short period of time to generate about 20 hp with 5 hp the engine overhead, 5 hp to the wheels, and 10 hp (7.45 kW) being stored in the traction battery. Roughly 25% of the energy runs the engine and 75% is used for traction power either when the engine is running or later from the traction battery.

Any vehicle that requires the engine to run at 30 mph, diesels, gas, or hybrids with little or no electric vehicle mode, will pay about 50% of their tank just turning over the engine. In contrast, a hybrid with significant electric vehicle mode gets a much improved efficiency, nearly double that of the other. Now granted, these numbers are approximate but they also match the specifications of the Toyota, Ford, and Honda hybrids and starter motor power needed by diesels and ordinary gas engine vehicles.

So I can admire BMW's efforts to improve vehicle, transmission and engine efficiency. But until they understand the 30 mph challenge, their results will remain as disappointing in the future as they are today.

Bob Wilson

Isn't that a reason why P&G works?

Yes.

I think BMW understands the problem very clearly but these are 300hp engines they are talking about. This isn't the base model poverty spec 3 series. If you are just interested in fuel economy then get the 4 cyl diesel with 116hp.
BMW UK : The new BMW 316d Saloon
Obviously Americans buying bmws couldn't care less about FE so the model isn't offered in the US. But believe me, BMW gets it.

316d 52.3 mpg US combined city/highway
116d 53.5 mpg US combined city/highway

P&G works especially well in traffic. <GRINS>

After all, no one would exceed a posted speed limit so the urban P&G wizard pulses to the speed limit, 30 mph, and then 'coasts' or 'glides' down to some lower value ... to the great delight of the other drivers. Then they pulse again to the 30 mph speed limit and again 'coasts' or 'glides' or 'feathers' or whatever down to their preferred lower speed. Again, to the great delight and enjoyment of the other traffic. But in no case is the P&G driver achieving an average speed of 30 mph because the glide is always under 30 mph.

So what are the pulse and glide parameters to sustain 30 mph?

• 10 mph dV - 35 mph to 25 mph
• 20 mph dV - 40 mph to 21-22(*) mph

In the first case, the driver has to be on a 35 mph street to average 30 mph (see title of thread.) In the second case, the driver has to be on a 40 mph street to average 30 mph. ... Have I missed something?

Surely no one is advocating exceeding the speed limit since that would risk getting a ticket. But driving at an average speed of 30 mph on roads posted at 35 and 40 mph is the great delight of other drivers who will announce their joy with horn salutes and raised fingers. <GRINS>

In all seriousness, the hybrid traction battery is another way of storing energy that does not depend upon the vehicle velocity. Unlike changing the velocity with other traffic around, the hybrid battery is an 'electronic' system that allows the vehicle to work like all of the cars surrounding it and not pose a very real collision risk or traffic obstruction.

Bob Wilson

* - due to non-linear drag effects, primarily aerodynamic, the lower glide limit has to be raised to compensate for the higher speed pulse.

The vehicle efficiency model works the same regardless of the engine size. Although the larger engine is likely to have greater internal drag and require more than 5 hp (3.7 kW) for internal engine operating losses, it still has to turn to maintain 30 mph and generate the fixed internal energy losses as well as the 5 hp needed to sustain 30 mph.

Bob Wilson

And don't forget hills. It's possible to use them to your advantage.

And when you get your hill, be sure it is a good one and use your 'hill hook' to hold it front:
http://hiwaay.net/~bzwilson/prius/coasting.jpg

Since you've proposed cartoon physics:
http://hiwaay.net/~bzwilson/prius/pri_mag.jpg

Of course if you want something more practical:
http://hiwaay.net/~bzwilson/prius/pri_tow_140.jpg

Bob Wilson

I'm sorry. You don't know what you're talking about.
:thumbup:
Justin

I regularly slow down to 5 below the limit or even 10 below the limit when going uphill to avoid accidental speeding when going back downhill. Other drivers just pass and I pretty much just ignore them.