mpg on long trips

[jaapv]

That's being a bit pathetic, the sentence says "almost free electricity". Obviously the meaning being that the engine is running anyway and driving the front wheels, any surplus power can put charge back into the battery to be used later when required at lower speeds. In the 18 months that I have owned this car I have found this to be far more efficient than using the battery up first and then just leaving it to do it's own thing, also if you keep the charge level up then you have full power if you need it.

the term surplus power is meaningless.
The engine will always output the amount of power demanded and take the amount of fuel needed at the particular energy level. Not more, not less. If you demand more power by charging it will use more petrol.

 

[twosout]

Remember there’s no such thing as a “free lunch”!
You might be right to describe regenerative breaking as providing “almost free electricity”, though, as it would otherwise be wasted.
An important thing to recognise is that the electricity provided by the ICE is not as cheap as the electricity you get from a socket. Fuel and electricity cost a similar amount per kWh but the efficiency of generation through an ICE is woeful (only about 20%).
If you have solar power at home, you might regard that as “free”, so long as there was not an alternative use for it.
The benefit of hybrid is that the ICE can be run at optimal efficiency, when in series, but at the expense of lugging extra weight around.

 

[cornclose]

You might be right to describe regenerative breaking as providing “almost free electricity”, though, as it would otherwise be wasted.

..which is why regeneration is wasteful when compared to coasting further without it.

 

[greendwarf]

the term surplus power is meaningless.
The engine will always output the amount of power demanded and take the amount of fuel needed at the particular energy level. Not more, not less. If you demand more power by charging it will use more petrol.[/quote]

Is this strictly true even for us down here in steerage with our Gx3h (no electric heater)? I doubt the control is sophisticated enough to only produce "heating" energy. I've always assumed it is better to use the heater (in winter, of course :lol: ) when using Charge or otherwise burning petrol and vice a versa (unless you will arrive with battery "left over") :mrgreen:

 

[dogman12]

Theory and opinion mean less to me than real world results. I would be happy to join in on test trips, of equal distance and conditions as equal as possible, to record the mpg achieved in "Charge" vs other modes, and estimate the cost per kwh. There will be a fairly large uncertainty because knowing precisely how many kwh were gained is not possible, at least with only the vehicle gauges. With one of the apps it would probably be better.

That said, I observed that on Charge it took about two miles at 45 mph to gain 1 bar on the battery readout. Anyway, I did not mean to impugn anyone's judgement.

 

[ThudnBlundr]

the term surplus power is meaningless.
The engine will always output the amount of power demanded and take the amount of fuel needed at the particular energy level. Not more, not less. If you demand more power by charging it will use more petrol.

Not really. The engine running at part-throttle is considerably less efficient than when it is running with an open throttle. So the amount of power produced per litre of fuel is far less. If you can burn that litre of fuel more efficiently, and harness that surplus energy to charge the battery, I'd argue you've got something for nothing

 

[jaapv]

That is true - in the sense that engine revs mostly determine the efficiency, not the amount of fuel you pump in. I objected to the suggestion that the power overhead is "free".

 

[ChrisMiller]

The engine running at part-throttle is considerably less efficient than when it is running with an open throttle. So the amount of power produced per litre of fuel is far less.

I don't see why the difference would be "considerable". A litre of petrol produces ~10 kWh of energy when fully combusted, so the only way to obtain less usable energy per litre would be to throw some of the petrol out of the exhaust or produce substantially more waste heat.

I'm not arguing that there are no efficiency differences at different throttle settings, just that they are second order effects.

 

[maby]

The engine running at part-throttle is considerably less efficient than when it is running with an open throttle. So the amount of power produced per litre of fuel is far less.

I don't see why the difference would be "considerable". A litre of petrol produces ~10 kWh of energy when fully combusted, so the only way to obtain less usable energy per litre would be to throw some of the petrol out of the exhaust or produce substantially more waste heat.

I'm not arguing that there are no efficiency differences at different throttle settings, just that they are second order effects.

I think it is more than I would consider a "second order effect" - fuel economy in most cars will peak somewhere around 60mph with a significant drop off on either side. The PHEV has the ability to improve fuel efficiency while in series (serial?) mode by adjusting the engine speed and routing any "surplace" power to the battery. In parallel mode, it can also improve fuel efficiency by adjusting the charge current to the battery and hence the load on the petrol engine - even if it is not possible to adjust the engine speed because that is determined by the required road speed.

 

[STS134]

That is true - in the sense that engine revs mostly determine the efficiency, not the amount of fuel you pump in. I objected to the suggestion that the power overhead is "free".

Well...no. Engine revs AND load determine the efficiency. This is called brake specific fuel consumption and is plotted on a BSFC map, like this: https://en.wikipedia.org/wiki/Consumption_map

View attachment BSFC.png

As you can see in the example image, this particular engine is most efficient when producing between 30-70 HP. But there are a lot of ways you can produce this amount of power: with low load at high RPMs, and with high load at low RPMs. It is the combination of producing about 45 HP @ 2500 RPM, which is a fairly high load, which maximizes the BSFC.

Now the problem with the PHEV is that it lacks a mechanical transmission, and thus, lacks the ability to vary the ratio of engine speed to wheel speed above 78 mph. So, the engine revs are absolutely FIXED compared to wheel speed above this vehicle speed. And this is where we get problems with the mpg on long trips, which is the topic of this thread. Imagine you're cruising along at 90 mph and you need the engine to be producing about 50 HP to maintain the vehicle's speed. As mentioned previously, there are a lot of ways to get this power output, as you can see on the plot. A normal tranny is going to put the engine into the lower RPM ranges to maximize BSFC. If more power is required, like during acceleration, THEN the transmission will vary the gear ratio to allow it to produce more power. The problem in the PHEV is that it can't vary the ratio. At 90 mph, the engine speed is fixed at ~3700 RPM. Don't need a full 70-80 HP? Too bad. It operates down in that yellow region which is EXTREMELY inefficient. Now as mentioned, it will TRY to charge the batteries as a way to increase load on the engine, and store power for later. But what happens when the batteries are already fully charged and you're just cruising along because it is, after all a long trip? Oh, right. It just operates very inefficiently.

Jason of Engineering Explained talked about this topic extensively in one of his videos.
https://www.youtube.com/watch?v=iNspNdVkslA

 

[ChrisMiller]

Thanks STS134. I'm think we can all agree that if you're looking for an efficient long distance 90 mph cruiser, the PHEV ain't it (but I'm not sure that any 4x4 SUV will do a lot better). It's very difficult (that means "I don't know how" ) to find fuel economy figures for any car at motorway cruising (even legal (UK)) speeds.

 

[AndyInOz]

You might be right to describe regenerative breaking as providing “almost free electricity”, though, as it would otherwise be wasted.

..which is why regeneration is wasteful when compared to coasting further without it.

Except of course for keeping your speed manageable, and at the speed limit while descending hills...

 

[jaapv]

When coasting you will have to use your mechanical brakes to adapt to traffic. I wonder what is less efficient. Using the paddles for the purpose is too unpredictable.

 

[ThudnBlundr]

I'm not sure why some people think that you can only coast in B0 and that you have to slow down when lifting off in B5. You can coast in B5, and then have full regen when required. It's not one-pedal driving, but it's not bad.

Braking in B0 is almost totally mechanical, and using mechanical brakes wastes kinetic energy as heat, whereas regen converts most of it into electrical energy stored in the battery.

 

[Kesto]

I have tried again today 126 miles non stop 65 mph on one full charge , 44mpg on 18-19 model. I have tried this a few times and it is pretty consistent