Mazda Skyactiv-X: Compression Ignition and Superchargers

[Eunos_Cosmo]

I don't disagree with any of that, but that isn't what you said in the post I quoted

One small revision.

"A Skyactiv-D wouldn't be enough to maintain battery charge at cruising speed, but would be enough to extend the range of an electric truck by a useful amount provided a 45mph top speed for most of that extended range is acceptable".

 

[homeforsummer]

One small revision.

"A Skyactiv-D wouldn't be enough to maintain battery charge at cruising speed, but would be enough to extend the range of an electric truck by a useful amount provided a 45mph top speed for most of that extended range is acceptable".

That implies that the range-extender would be completely useless as soon as the speed rose above 45mph. Which isn't the case - it would still be charging the battery, just not at a rate enough to off-set the rate at which it was being depleted.

No need to experiment to prove this, as you should be able to imagine it. Which truck goes furthest?

The truck doing 60mph - above the 45mph "equilibrium point" - whose range-extender is running, or;
The truck doing 60mph - above the 45mph "equilibrium point" - whose range-extender is off?

Nobody is suggesting a 45mph speed is acceptable - that's just the point at which the engine is able to maintain a steady battery charge. It's not a black and white scenario - it's perfectly acceptable to have a truck doing 70mph which is depleting its battery (as all EVs do as they use energy), but doing so less quickly than if there was no range-extender at all...

 

[Eunos_Cosmo]

That implies that the range-extender would be completely useless as soon as the speed rose above 45mph. Which isn't the case - it would still be charging the battery, just not at a rate enough to off-set the rate at which it was being depleted.

No need to experiment to prove this, as you should be able to imagine it. Which truck goes furthest?

The truck doing 60mph - above the 45mph "equilibrium point" - whose range-extender is running, or;
The truck doing 60mph - above the 45mph "equilibrium point" - whose range-extender is off?

Nobody is suggesting a 45mph speed is acceptable - that's just the point at which the engine is able to maintain a steady battery charge. It's not a black and white scenario - it's perfectly acceptable to have a truck doing 70mph which is depleting its battery (as all EVs do as they use energy), but doing so less quickly than if there was no range-extender at all...

I see what you're saying now. Do most range extenders work like that? I thought they were designed so that the generator could basically provide enough power for sufficient performance until the fuel ran out. Maybe I remember the i3 not behaving like that...

You would basically subtract the max ICE output from the Electric motor draw for the net consumption. (216kwh-120kwh). Accounting for efficiency, that would be about 105kw/hr at 60mph assuming all factors described. That would get you about 4.75 hours of run time and 285 miles of range. You'd burn 42 gallons of fuel (probably less with the Mazda engine). Obviously anything less than max load on the ICE would decrease that range. So either:

45mph max with range limited by fuel (1650 miles with 300 gallons of fuel) -or-
60mph max with range limited by battery (285 miles w/ 500kwh battery) -or-
60mph max range for 285 miles and then reduced performance (slower acceleration/top speed) after that.

So maybe it's not such a bad idea after all.

 

[homeforsummer]

I see what you're saying now. Do most range extenders work like that? I thought they were designed so that the generator could basically provide enough power for sufficient performance until the fuel ran out. Maybe I remember the i3 not behaving like that...

What I'm describing is probably closest to something like the i3 than it is the Karma or a Volt, but it seems like the more logical solution for a larger vehicle designed with bigger distances in mind.

40-ish miles in a Volt with the option to do much more is fine, because most people don't drive very far on average, but large commercial vehicles typically travel much further on average so it makes sense to prioritise the cleanest power option (EV) in that scenario and simply use the range-extender to reduce the rate at which the battery depletes.

The i3 REX is I think about 50/50 on how far it'll go on EV/gasoline. Something like a Volt, with 40 miles of EV out of 420 miles altogether, is more like 10/90.

In a commercial application you'd probably want the range-extender running all the time - it'll still be more efficient than the huge inline-six or V8 diesel of an equivalent vehicle, but usefully improve the range an operator gets from a full charge. If it'll do 300 miles on a battery alone but you can extend that to say, 500 miles by running a fuel-efficient generator constantly, then you're still making massive savings over a regular truck doing those 300 or 500 miles.

 

[Keef]

Spinning a generator seems like it'd have lower load than the combined efforts of all that

The amount of work done doesn't care what's causing it. A dyno loads an engine 100% by doing the same thing a generator would do. 100% power is 100% power, doesn't matter what you're doing with it. And that's my point - an engine hauling a generator at 100% power is cranking some serious electricity. Imaging the alternator on your engine clamping down so hard that your engine is full throttle at its power peak and can't go any faster.

 

[homeforsummer]

The amount of work done doesn't care what's causing it. A dyno loads an engine 100% by doing the same thing a generator would do. 100% power is 100% power, doesn't matter what you're doing with it.

I didn't quite phrase it the way I meant to.

What I was getting at is that the efficiency of a Skyactiv-D in a car varies according to all sorts of different factors in context of moving a vehicle from point A to point B - gearing, rolling resistance, aerodynamic resistance, gravity.

An engine spinning at 4000rpm will use a set amount of fuel (all other factors being equal), but how efficiently that fuel is used depends on what you're doing with the output. If you're driving up a hill in second gear at 4000rpm the same amount of fuel will get you a shorter distance than if you're in sixth gear on a flat road at the same revs - and the same engine spinning a generator at the same speed will probably be an even more efficient use of energy for a given distance.

 

[Eunos_Cosmo]

I didn't quite phrase it the way I meant to.

What I was getting at is that the efficiency of a Skyactiv-D in a car varies according to all sorts of different factors in context of moving a vehicle from point A to point B - gearing, rolling resistance, aerodynamic resistance, gravity.

(A)An engine spinning at 4000rpm will use a set amount of fuel (all other factors being equal), but how efficiently that fuel is used depends on what you're doing with the output. (B)If you're driving up a hill in second gear at 4000rpm the same amount of fuel will get you a shorter distance than if you're in sixth gear on a flat road at the same revs - (C)and the same engine spinning a generator at the same speed will probably be an even more efficient use of energy for a given distance.

(A): Not necessarily true and i'm not sure what you mean by other factors being equal. An engine with zero load at 4,000rpm will barely use any fuel. An engine at 100% load at 4,000rpm will use a lot of fuel, relatively speaking

(B): That depends on quite a lot of factors. How steep the hill is, what the gearing is, what the total drag of the vehicle is. The vehicle on the hill is likely to consume more fuel, but I wouldn't say its impossible for it to consume less. If the total load of the aerodynamic drag (remember, it squares with velocity) is greater than the gravity load of the vehicle on the hill, its reasonable to imagine that the car on the highway has higher fuel consumption/shorter range.

(C): Again, engine speed has much less correlation to fuel consumption than load does. It's entirely possible that option C could be the least efficient use of the engine's output, in some situations, because you are converting kinetic energy to electric energy, and then converting electric energy to kinetic energy again. You will still have some parasitic loss through mechanical inefficiency just as the engine-driven car does, but you're also now introducing loss due to electric motor inefficiency.

Example:
Car 1: 100kw ICE (@ peak efficiency)
Car 2: 100kw ICE (@ peak efficiency) + 100kw EM

In car 1, assuming a modern front drive vehicle, you'll probably get to use about 90-95kw of that engine output for moving the vehicle, at peak efficiency.
In car 2, assuming some drivetrain loss + electric motor losses, you'll probably be getting less than 90kw of that engine output for moving the vehicle, at peak efficiency.

The difference is that car 1 will be operating at peak efficiency rarely, and car 2 could be operating at peak efficiency much more often, depending on a large variety of factors.


Anyone else hyped for the reveals later this month??

 

[homeforsummer]

(A): Not necessarily true and i'm not sure what you mean by other factors being equal. An engine with zero load at 4,000rpm will barely use any fuel. An engine at 100% load at 4,000rpm will use a lot of fuel, relatively speaking

That's what I get for posting just after waking up.

My "all other factors being equal" comment was referring to both load and throttle position. If both are the same but you're in second gear you're going to travel less far for a given quantity of fuel than you are in top.

(B): That depends on quite a lot of factors. How steep the hill is, what the gearing is, what the total drag of the vehicle is. The vehicle on the hill is likely to consume more fuel, but I wouldn't say its impossible for it to consume less. If the total load of the aerodynamic drag (remember, it squares with velocity) is greater than the gravity load of the vehicle on the hill, its reasonable to imagine that the car on the highway has higher fuel consumption/shorter range.

Hmm. While there are fringe scenarios, I'm not sure this is really correct.

I deliberately picked a very low gear and a very high one here to make my point, but this sort of thing is easy enough to corroborate using any modern car with an instant fuel computer. If I'm doing 30mph up an urban hill near my apartment economy instantly improves just by changing from second to third to fourth (in most cars this particular hill is a bit too steep for higher gears, though a few autos have shifted into sixth...)

I'd say my example here is relevant more often than it isn't. You'd have to pick a very particular set of extraneous factors for a car - and by and large we're still discussing a 2.2-litre Skyactiv-D here - to be more efficient in second gear at high revs going up a hill to be less fuel-efficient than it is at a fast cruise on the flat, even considering aero drag.

(C): Again, engine speed has much less correlation to fuel consumption than load does. It's entirely possible that option C could be the least efficient use of the engine's output, in some situations, because you are converting kinetic energy to electric energy, and then converting electric energy to kinetic energy again. You will still have some parasitic loss through mechanical inefficiency just as the engine-driven car does, but you're also now introducing loss due to electric motor inefficiency.

Again, not so sure about this. Mostly because "electric motor inefficiency" is very low indeed - refer back to my comments about thermal efficiency in engines and electric motors. Drivetrain losses aren't the only factor in internal combustion - you're only getting about 40% of the energy from the fuel itself - the rest is being wasted as heat and noise.

And while I plucked 4000rpm out of the air somewhat, a diesel engine running at constant RPM should in theory be more efficient than one used in a conventional vehicle where it's facing varying load, varying revs etc. Engines used as range-extenders tend to be set up to run at a constant rpm - the rpm where they're doing the most amount of work in the most efficient manner.

If you knew that your engine only ever had to run at a constant rpm (and constant load - you're running a generator, not some wheels through various ratios that have to accelerate, slow down, and go up and downhill), you'd likely get even better thermal efficiency from it than you can currently, since everything could be changed to maximise efficiency at that one speed - from fuelling, to the shape of the combustion chamber and length and shape of the intake and exhaust systems.

Anyone else hyped for the reveals later this month??

Going back to the subject of the thread... yes! Whether drivetrains or design, Mazda is still one of the more exciting car companies around at the moment.

 

[VXR]

It would cost more in the beginning, but surely the battery could form the rolling stock area of the trailer too? It would help lower the centre of gravity a bit, too.

 

[CodeRedR51]

Mazda Skyactiv-X Review | The revolution begins with a squeeze-bang

How Mazda got Skyactiv-X to work is incredible

 

[Obelisk]

Mazda Skyactiv-X Review | The revolution begins with a squeeze-bang

How Mazda got Skyactiv-X to work is incredible

That second article was an excellent read. Thank you for sharing!

 

[Omnis]

I'm over SkyActiv-X at this point. I think Achates's solution is much more elegant. And you get the same economy as SkyActiv-X while making 400 horsepower and 500 torques. Their unit in the Aramco-sponsored truck was built with off-the-shelf parts. Imagine the same technology with custom-engineered packaging. 2020 is going to be an ICE renaissance.