Diesel economy, and math.
 

A litre of diesel is the energy equivalent of approximately 11 kWh.

If you get 25 mpg that means it "costs" you 440 watt hours to go one mile.

Chances are, the lion's share of this is going to go on a mixture of;

1. rolling resistance
2. aerodynamic resistance
3. acceleration

#1 is affected by good maintenance, and custom options such as wide or narrow tyres.

#2 is affected by vehicle speed, not much else you can do to change it.

#3 is affected by vehicle weight, and jackrabbit starts.

Once you own a vehicle, there isn't much you can do about air drag coefficients, or vehicle weight, they are pretty much fixed.

Once you own a vehicle, you CAN make a huge difference to actual air drag by keeping the speed down, 60 mph or less, you can also make a huge difference by driving smooooooth and steady and reading the road ahead.

Once you own a vehicle you can minimise rolling resistance by maintaining the vehicle running gear well, and fitting narrow tyres.

You can boost your economy by 30% doing these things.

Making SIGNIFICANT changes is fuel economy can only be done with a NEW vehicle.

It needs to be very low drag, which means low.
It needs to be VERY light, think 750 kg kerbside weight.
It needs to be SLOW, think 0-60 of 15 second and a top speed of 70 mph

Do this and you can get a 100 mpg diesel car.

Or go from 440 watt hours per mile travelled to 150 watt hours per mile travelled.

But you absolutely CANNOT do it with a vehicle built like a modern car, steel body, glass windows, leather seats, 2,800 kg kerbside weight, fat tyres, 125 mph top speed, etc.

In the end, it is all going to come down to cost.

Here in the UK one kWh cost around 13 pence, that's around 20 cents, doesn't matter how you buy it, as electric, as diesel, as natural gas.

The amount of kWh that your weekly wage can afford is going to decide what kind of economy your vehicle runs.

You are absolutely correct; and when the cost of a kW-hr of energy doubles or triples some of those changes are likely to be implemented in production vehicles.

I disagree. Most of that energy is lost in heat, ie exhuast gas. Also, radiator cooling. Heat is energy. Energy is not created nor destroyed, just transformed.
Electricity can be very effiecient. I think some electrical motors get 80 to 90% effiecient. You do have loses by transmitting electricity over power lines. You also loose when you transform from one voltage to another.
Tom

Interesting read. I enjoyed it.
Here's my take on the plug-in vehicle thing. I haven't done any calculations but the thought process is as follows:
In a conventional IC engine you take a fuel which has chemical energy and transform it into heat energy which you then transform into mechanical energy. Each of these steps involves a loss of energy which is quantified as efficiency.
In a plug-in vehicle you take a fuel and extract it's chemical energy to produce heat energy to produce mechanical energy to produce electricity which is transported through several step-down transformers to your house. You then convert that to chemical energy by charging a battery which is used to drive a motor thereby converting it to mechanical energy. Each of these steps also involves a loss of energy. One could infer that a greater number of energy changes would involve a greater loss of overall efficiency.
However, the scale of power generation (and the vast array of heat-recovery features in these plants) makes it quite efficient and the heat losses in an IC engine make it quite inefficient. Also, from the first post, the desire to have it all in one package (speed, comfort, safety, convenience) tends to reduce the efficiency of that package.
I don't know the answer but it's interesting to think about. I do think that if we eliminated all drive-up windows (my pet peeve) and built a feature into cars that minimized the time you could idle them it would save a bunch of fuel. And the bottom line is we are going to have to be better stewards of energy as the population grows and the planet does not.
Thoughts?

From what I read many of the new cars will turn the engine off each time you come to a stop of some minimum interval of time. When you hit the gas again, the motor comes back to life - even in gas engines. It is hard for me to understand how so much energy can be wasted at stop lights that this is an efficient answer. Doesn't this strategy play havoc with the starter wear and maybe the battery? Are the overall savings justified?

You have clearly never put your hands on a 75 kW / 100 BHP electric motor, or control electronics, or any DC battery bank (of any variety) producing that sort of power....

80% overall system efficiency is a laboratory dream.

It is what is claimed for brushless motors. Cobalt motors at claimed at above 80%.
Tom

I think his point was that automotive ICEs are closer to 15% while stationary power plants can convert thermal to electrical energy at closer to 40%.

Also, depends on what cycle the ICE power plant is in. At Highway speeds, they do well against electric power or Hybrids. Deisel is more effieceint that gasoline, but gasoline has made strides. Also MB and others are messing with Atuodeisel (??). Basically a deisel engine designed to run on gasoline. I think the main savingins is in emmisions add ons.
Tom

Agreed, I'm just using round numbers. It is difficult to talk about cost per unit energy only, it depends on the form of energy.

meh, might as well claim that my hydraulic transmission is 95% efficient, it is meaningless unless you take the system as a whole, that means....

1. MEAN efficiency of electric motor, likely to be around 50% for any motor with a peak efficiency of 80%
2. MEAN efficiency of control circuitry, usually around 85%
3. MEAN efficiency of battery on discharge cycle, 50% if you are lucky.

Resulting on an overall efficiency of 21.25%

You STILL have issues with aerodynamic drag and rolling resistance.

You ALSO have the downside of the mass of batteries you are toting around, which weigh the same discharged as fully charged, with an energy density of approx 150 watt hours per kilogramme... you get...

4 kilogrammes of battery per mile travelled, so a 100 mile range = 400 kilogrammes of battery.

while the diesel car will do 100 miles on 4 kg of diesel.

100 kg is 7.5% of the weight of a small/average vehicle kerb weight, which therefore represents 7.5% of rolling resistance, which, due to the nature of the battery, IS ALWAYS THERE.

THAT is precisely my point, if you do the actual math and actual engineering, it doesn't, all that matters is the cost of the energy per kWh.

What you are missing is the fact that the energy cost increases as it is converted to a more useable form. A pile of coal containing 1 kW-hr of energy is much less valuable than 1 kW-hr of electricity delivered to your house; it will take more than 2 kW-hr of "coal energy" to make 1 kW-hr of "electrical energy" after a very expensive conversion process. That is what you a paying for; energy delivered in a usable form.

True. I think think electic vehicles are in their infancy. If/when lithium batteries become viable, then the weight paragram might change. Although different, I fly RC airplanes. 10 years ago, electrics were a niche market. Now, every Hobby shop, Walmart, K-Mart etc. has them. The performance, cost/weight of batteries, everything has changed.
I'm not saying diesel is dead (I hope not, it keep shoes on my feet). Presently, I don't see the cost as being practical. Niether was ABS, or air bags originally. I do agree that the accounting of mpg might and probably is deceptive on these vehilces, ie Chevy Volt type.
Also, I'd rather put my hand on an eletric motor, than the exhuast of an ICE motor. HEat loss is every loss. Check out semi adiabatic motors.
Tom

bzzzzzzzzt

compare like for like, ice exhaust vs leccy motor windings.

To most extent. I think it boils down to effeicency of transfering energy and practicallity of doing it. Also with ICE, what is the energy used to transport the gas or diesel to the car? If we are using that as a measure for electricity, then that needs to factor in for petrol. Maybe we do find deisel is the mmost effienceint.
To me the prospect of diesel is that of Bio fuel. I think it is the most promissing form of it, for transportation. Electricity from wind, hydro, solar is probably best used for residential and comercial HVAC and lighting, or stationary usage. At least presently.
Tom

Are you prepared to stick your hands in the combustion process? As difficult as that may be. Or go Tunisian?
Tom

Skinny tires cost lives. My 2 cents.

LOL QOD

[QUOTE=custom options such as wide or narrow tyres..[/QUOTE]

In my 603960 (6cyl turbo) the best I have achieved is 35mpg (american) but when I put on a set of bigger winter tyres, it dropped 10%. I was surprised as I was expecting the larger diameter tyre to give a small increase in mpg.
That said, the increased grip is worth far more than the cost, here in the sloppy snow of eastern Canada.

I enjoy what you're trying to say, but instead of sounding like a troll why don't you explain why his point is wrong.


The best part about diesel engines is how simple it is to make a compatible biofuel. I can't wait for the hemp industry in this country to take off and start using that as a source of biodiesel.

1. Starting a hot engine is real easy, easier still if the car is rolling. Which hits a battery harder? .5 seconds at 200 amps for a starter or a 10 second cycle of 10 amps for a cooling fan?
2. Full authority ECU's (push button start) make this more practical.
3. At idle, gas engines suck the air charge past a nearly closed throttle, doing significant back work. Diesels idle by governing down to idle stop or idle rpm w/ free flowing intake. Stop/start engines gain more w/ gas than diesel.
4. The benefit varies- you would see none on the thruway, but a lot in a traffic jam. Most people live in cities, most urban drivers know sitting in traffic.
5. Stop/start engines would also benefit air quality & reduce heat pollution where it sucks the most (well, due to cars)- next to jammed up urban traffic.

Then factor in the 80% efficiency of grid power distribution from power station to suburb, the 88% efficiency of the substation, the 85% efficiency of the local grid, the 88% efficiency of the local step down xformers, and the 90% efficiency of the last mile low voltage supply to the home (actual CEGB numbers) for an overall efficiency of 47%

Are you prepared to stick your hands on the 92 VDC terminals of a leccy/hybrid car battery bank?

like for like, it's a *****.

Larger diameter tire == Odometer offset.

I recently got 33.4 on a tank with 2 adults, 2 kids and the trunk full of wheels. Avgd ~80mph.

your point #5 is the ONLY reason it is done.

200 amperes x 12 volts = 2.4 kW, 2.4 kW for 10 seconds = 7 watt hours. BUT, you have to recharge the battery, between alternator and battery inefficiencies you are up to 30 watt hours.... this can ONLY come from energy that would otherwise be used to propel the car.

A diesel idling at (mb 300d) 0.6 litres an hour = 6.6 kW an hour = 110 watt hours for 1 minute idle.

If you work it out as 20 traffic light stops on a journey, each one for a full minute, you save around 2 kWh, or around a pint and a half of diesel.

but in the real world, your radio is running, your electric cooling fan may still be running, your heater / ac may still be running, and these are all sucking the battery down, so your real comparison is not 30 watt hours vs 110 watt hours per stop, but 60 or 90 watt hours vs 110, plus the increased wear and tear and load on starter, alternator, battery (only so many charge / discharge cycles in any battery) and suddenly it looks crap from an energy viewpoint.

Even emissions it only works if you ONLY measure emissions while waiting at the stop light.... you get MORE emissions on the way to the next stop light recharging the battery.

Good, now we are half way there. What's the efficiency of an ICE motor? I do agree with you assesment of efficencies of transforming from one voltage to the next. Still, you have to figure in the efficiency of transporting liquid, ie gas or diesel. Pipe lines do it fairly effiecently to a point. That is part of the problem with Ethanol. I don't think you can use the same pipe lines with certain types of fuel.
Tom

Also, he is essentially regearing the drive train. A lot of Jeeper have problems when the put on bigger tires, and don't regear the axle(s). Also, changing the gear in the speedometer cable. So, the gear train isn't set up to effieciently power the wheels. Motor bogs so it doesn't operate as efficiently.
Tom

10 seconds goes with the 10A cooling fan, not the 200 amp crank, I guessed .5 seconds, I doubt it takes that, it'll hit on the first power stroke, if it's hot.

The prius uses the gearbox to crank it's engine if it's moving. Quite a neat bit of controls, actually.

If it increases mileage, and you don't plug it in at the end of the day, I'm guessing it looks alright from an energy viewpoint.

The last rental car I had in Europe had this feature... Audi A4 2.0L TDI. Pretty neat and you could turn it off. WHen the motor was cold, the feature automatically was turned off. It wasn't till the motor warmed up did it automatically shut off when stopped for a more than a few seconds in neutral. Press the clutch and it immediately started up before you got it into first and off you went.

I don't doubt that all those additional starts put a strain on the starter, but in terms of the life of the vehicle... I don't think it would be a big deal.

Ooops, yes you're right, the odometer offset accounts for 4%. Tyre size changed from 195/65-15 to 215/65-15. This car can easily pull the higher gearing so I don't think engine fuel efficiency is affected, but rolling resistance is significantly higher with the bigger tyres.