How far travelling a Hybrid with no petrol

Find a different garage - they are talking bollocks:

One mile and a little bit, depending how careful you are. The battery is really quite small and has less than 1kwh available from full charge, and the computer would not let you seriously flatten it.

That's about right, when on the flat. I find my Auris kicks out of battery mode after that sort of distance (or when going over about

I'm not sure I understand the logic in Toyota's statement "Although our full-Hybrids are designed to operate in electric-only modes, they have not been designed to run without petrol. Doing so could cause severe damage to the Hybrid system." Clearly the batteries and motor are powerful enough to be the only form of propulsion (eg in towns where pollution is more of a problem), so what's the big deal about running out of petrol It's only doing the same as normal electric-only mode: running without the petrol engine. Obviously you need to limit how far you drive like this before filling up, to avoid flattening the battery, but I presume there is an estimated range which takes account of current battery charge and current petrol level, and adds the two together, making any adjustments as petrol is burned to top up the battery, or as the battery is discharged in no-petrol zones.

I presume if you are calculating mpg figures you need to average over several tank-fillings to take into account that the battery may be in a different state of charge on each occasion that you fill the tank, and to minimise the effect of any differences in charge on the mpg calculations.

What? Is that all the charge that it holds? What's the point in having such a small battery? I though the whole idea of hybrid cars was that the engine charged the battery (as well as propelling the car) when you are out of town, so as to be able to drive the car in town on battery only. And that may be a significant distance from "cleaner" countryside and suburbs into "polluted" city centre.

If the car has a range of only a mile or so on battery, how is a hybrid any better than a petrol-mechanical car where the engine runs all the time. I presume hybrids like the Toyota Yaris are petrol-mechanical rather than petrol-generator-motor during the times that the engine is running - ie that there is a mechanical transmission (whether manual, torque converter or CVT).

A ballpark is 400Wh per mile, so do the sums based on the battery capacity. Depending on the technology you might only get 70% usable capacity because the best battery lifetime is avoiding it getting too empty or too full.

Theo

The battery smooths the operating curve, enabling the engine to run at its optimum point. For instance, if you're in stop/start traffic in a petrol manual, you're continually flipping between 1st, 2nd and stopping, revving up and down while making very little progress.

In a hybrid, you're doing all the stop/start on the battery. Eventually the battery will deplete and the engine will kick in to charge the battery (even when stationary). It might run a lot faster than idling to do that at the optimum point. Once the battery is charged it'll stop and you'll have another 20 mins (or whatever) of electric mode.

Another reason is regenerative braking: when in stop/start, it recovers some proportion of the energy back into the battery. As well as more efficient, this saves wear on the brake pads. On many hybrids they're never changed.

Addiitonally, electric and engine can be used together to increase power - eg when flooring it joining a motorway, the electric motor provides additional torque for acceleration. That again means the engine is running optimally, and so a smaller engine can be fitted. That improves efficiency because you don't have to use the bigger engine the rest of the time when you don't need the extra power. Having electric motors (there are usually two) also means the gearing is a lot simpler - just one three-part planetary gear set on many cars.

If you have a PHEV then they have bigger batteries to make it worthwhile charging from the mains. But basic hybrids are using the battery for smoothing rather than pure electric drive.

IME, the only time pure-electric mode is worth using if there's a time you know better than the car - eg you're only going a few hundred yards and don't want the car to start the engine because you'll turn it off in a minute. Most of the time letting the car handle mixing power modes is more efficient.

Theo

My spreadsheet graphs the fill-to-fill MPG, the long term MPG (since vehicle purchase) and the average of the last five fillings MPG.

Since one can only go a mile or so on the battery, I suspect the state of charge at fill time has little effect on the figures.

I don't remember the proper term for the gear-box type but it can accept two sources of input power (from the petrol engine and the electric motor) and feed that to the wheels.

We are (half) looking at petrol hybrids at the moment. The two we have filtered it down to seem to claim about 30 miles in one case and 17 miles in the other, although they are both 4x4s so not exactly light cars.

We looked at an all electric 4x4 (the Tesla) but ruled it out due to concerns over getting caught out with a 'flat' battery. The story they tell is all well an good until you factor in finding the 'top up' station has a queue and your brief stop for a coffee turns into dinner.

Or, as happened to one of my Tesla owning friends yesterday, you forget to charge it and a minor domestic emergency arises, leading to a desperate search for the nearest "supercharger". (Grrrr. That word already has a perfectly good meaning.)

The computer shows you lifetime fuel average

Getting moving is a large drain on power, an electric motor is good at this, driving in town the battery gets a lot of recharge from the slowing down regeneration, around town the yaris hybrid gets about 65 to the gallon because of the elctric assistance, running on a motorway you will get a lot less.

sounds about right, the battery is 0.95kwh

In my Auris, that figure appears to be optimistic.

Depends how you drive. These days I tend to go more slowly on motorways than heretofore, so as to avoid the yo-yos.

The computer monitors the state of the battery and the engine, turning on one, other or both as appropriate for the conditions. When in electric mode the engine is always available within about a second if the conditions require extra power, or the battery is becoming unhappy. If you don't have the engine available because there is no fuel, it can put the electric system out of spec - eg stressing the motor trying to draw more power out of it than it's comfortable with, or draining the battery into a region that causes lasting damage.

The systems are extremely conservative: the primary goal is to not kill you. So if you floor it, it will assume you need the power to get out of a tricky situation - it'll deliver as much as the physics will allow, and damage to the car is a secondary concern. That's why you need both systems functional, because each system is the backup for the other.

Unless it's a PHEV it's not really worth bothering with. I don't know if the onboard PHEV systems keep track of that.

Theo

I managed to raise the lifetime average of a Prius by 1mpg on a long motorway run, I used the cruise control for braking whenever it was feasible. I was running at 70 - 80mph most of the time and it crept up to 59.something

Not so. The original Prius only moved a few feet on battery only before the engine starts. The main idea is not to have the engine idling when not needed. And creeping in heavy traffic.

It is why they were only more economical at very low speeds in heavy traffic. In light traffic they can use more fuel than an equivalent petrol only car. And if driven hard with a full load on the open road can be rather worse.