12 Comments

1. NickM says:
   June 12, 2011 at 11:10 pm

   OK, this has been bugging me for a while now…

   Hybrid cars claim better mpg than petrol cars of the same size. How can this be?

   By hybrid I mean “pure” hybrid in the sense that the only energy that goes in is petrol.

   Now obviously electric motors are more efficient thermodynamically than petrol engines but the electric motor is operating on energy generated by burning petrol so…

   I just don’t see how this is thermodynamically possible. I seem to recall from A-level physics that an electric motor is typically about 80% efficient. So one would naively expect all other things being equal a hybrid to be 80% as efficient as a pure internal combustion engine driven car. Indeed less than that due to the not inconsiderable mass of the second drive train.

   Ah, but regenerative braking! Hmm… How much does that actually save and surely that has much to do with the type of driving involved. It is obviously vastly more of a factor in city driving than motorway cruising. And of course driving style and technique is an issue here.

   Related to regenerative braking - what does that do to the state of the cells? I only own one thing where the manual says it’s fine and dandy to recharge the battery howeverwise. That’s my very new tech Sony DSLT camera (Alpha-55) and that has a spanky new Lithium polymer cell.

   Is there an element of smoke and mirrors to hybrids? I do wonder not least because I gave an undergraduate presentation on a rather curious interpretation of fuel consumption for motor vehicles* and I came across a headslap factoid. Diesels tend to do better mpg than petrols because we think of car fuel in terms of volume rather than mass and diesel is of course denser than petrol.

   My question relates entirely to the operation of such vehicles and not to the energy used in manufacture which I am given to understand is considerably more for a hybrid. Yes, there are a lot of issues in the manufacture, disposal and the commodity prices of stuff you make the electrics out of and all that but that’s another question.

   Please help. It’s a long term itch.

   *It’s quite cute but I’d be digressing if I went into it here and more to the point that was in 1993 so I’d have to don my remembering cap.
2. Willy Wanker says:
   June 12, 2011 at 11:46 pm

   Has the ceiling cat seen you masturbating?
   Inquiring minds need to know.
3. CountingCats says:
   June 13, 2011 at 12:50 am

   Willy,

   Fuck off and don’t come back.
4. JonB says:
   June 13, 2011 at 5:58 am

   Nick, I don’t really have the numbers to hand, but I can probably help you a bit on the hybrid question. Comes down to flattening out the power utilisation really.

   An IC engine petrol engine might make 30% thermal efficiency on a good day with a decent design. A big marine diesel (the sort you’ll find in container ships) will knock on the door of 50%. Ok, but, as you say, all the energy is coming from the engine anyway, so its absolute efficiency is irrelevant. What is relevant is its efficiency when doing bugger all … and when doing not very much. If I’m sitting at the lights, engine running, I am by definition 0% efficient from the point of view of turning fuel into miles covered.

   Petrol engines simply aren’t very efficient at part load. Take a very average petrol engined Ford Focus - at 60mph it’s not using all of its 120-odd hp - having tried to Vmax one once upon a time I’d guess it might be using 15hp. Now, if I assume the torque the engine is capable of generating is linear across this part of the rev range (incorrect assumption, but we’ll run with it for now) and I’m at half the speed at which maximum power is generated, then I’ve got about 60hp actually available - of which I’m using 15.

   Power generated in a petrol engine is roughly proportional to air moved through it, you burn as much fuel as you have air to burn it with (most of the time), so to get down to 15hp I’m using the accelerator to literally throttle the air intake. This leads to pumping losses, the engine is having to do more actual work to draw less air in (less work to compress it too, but I’m getting out less power because I can’t burn as much fuel).

   All this is a really long winded way of saying that petrol engines are bloody inefficient at part load. They’re not very efficient flat out either, because the air/fuel ratio gets monkeyed with to maximise power at the expense of efficiency when you’re at full throttle and high revs, plus you’re now doing one-hundred-and-silly mph and beating yourself to death against aerodynamics.

   Hybrids attack this problem from both ends. One, they eliminate as much of the minimal-load part of the cycle as possible, by doing it on the batteries. When you’re at moderate load (but too much for the batteries to do alone) then they make use of spare engine capacity by charging the batteries (that you just drained pootling around). You also make the engine smaller, so it gets up to high utilisation sooner, and you stop monkeying with the air/fuel ratio at the top end, and leave the engine to be boringly efficient. At this point you have a slow underpowered beast, but most people don’t drive their conventional cars around at full throttle all day anyway, so they fill in the maximum power end of the curve with the batteries and motor to give you a temporary power boost.

   And if your batteries and motor are efficient enough, then this works out more efficient than just having the petrol engine alone. I think a well designed and controlled traction motor is probably more like 90% efficient. The regenerative braking is a nice bonus, but it’s less important than improving the utilisation of the engine. More of the charging is probably done in slow time when the engine’s running at moderate power than in sudden bursts of braking. To avoid battery abuse I’d guess they limit the regen power flow, or maybe even use caps.

   unless you spend all your time at speed and that poor little engine spends all its time running flat out, you run out of battery boost, and you have the Top Gear Prius vs M3 track test. The M3 will have been running its engine at low-ish revs but a wide throttle, most efficient part of the operating curve. The Prius was being hammered, and so it suffered.

   On the other hand, you could just buy a diesel, which doesn’t rely on a linear relationship between air and fuel, so is never throttled at all.

   Diesel is more energy dense, yes, and the ideal cycle is inherently less efficient, but high compression (more importantly high expansion) is good for efficiency, and diesels can run much higher compression than petrol engines. Too much compression on a petrol engine and you start to get detonation (premature combustion, basically), which is exactly as bad for the engine as it sounds. Diesels squirt the fuel in when they want it to burn, and avoid that problem.
5. DeNihilist says:
   June 13, 2011 at 3:07 pm

   OH my friends from the great race of Richard the Lion Hearted, please say it aint so!?!?!?!?!?!?!?!?

   http://www.dailymail.co.uk/news/article-2002605/RAF-pilots-language-lessons-use-French-aircraft-carrier.html
6. CountingCats says:
   June 13, 2011 at 5:29 pm

   Um, Richard Coeur de Lion was Norman. He spoke French as a matter of course and didn’t even know English.
7. NickM says:
   June 13, 2011 at 8:40 pm

   Cats said it.

   He also spent approximately six months of his reign in England and all of that was taken-up raising money for foreign adventures. Richard Coeur de Blair?

   From his wikipedia entry…

   When he was crowned, Richard barred all Jews and women from the ceremony, but some Jewish leaders arrived to present gifts for the new king. According to Ralph of Diceto, Richard’s courtiers stripped and flogged the Jews, then flung them out of court. When a rumour spread that Richard had ordered all Jews to be killed, the people of London began a massacre. Many Jews were beaten to death, robbed, and burned alive. Many Jewish homes were burned down, and several Jews were forcibly baptised.

   The later is of course also more usually now known as “drowned”.

   He then managed to piss off the Duke of Austria by, following the siege of Acre, having his men throw the Austrian flag in the moat. Alas he then fell into Ausrian hands. The Duke of Austria was a vassal of the Holy Roman Emperor… wikipedia again…

   The emperor demanded that 150,000 marks (65,000 pounds of silver) be delivered to him before he would release the king, the same amount raised by the Saladin tithe only a few years earlier, and 2–3 times the annual income for the English Crown under Richard. Eleanor of Aquitaine worked to raise the ransom. Both clergy and laymen were taxed for a quarter of the value of their property, the gold and silver treasures of the churches were confiscated, and money was raised from the scutage and the carucage taxes.

   So apart from being French, anti-semitic, addicted to war, having a hell of a temper on him, milking the country dry and then bankrupting it he was a great Englishman.
8. NickM says:
   June 13, 2011 at 8:48 pm

   JonB,
   Thanks a lot! That all makes perfect sense to me.
9. CountingCats says:
   June 13, 2011 at 9:15 pm

   In the context of his times can’t really criticise the bloke for anti semitism. Back then everyone was. Can call him a dick for everything else tho. Some rulers ruled and didn’t bankrupt their countries.

   The Robin Hood legends get him completely wrong.
10. NickM says:
    June 13, 2011 at 10:32 pm

    Cats,
    I’m actually not 100% you’re right on the anti-Semitism thing here. Yes, the Jews tended to be distrusted and disliked and here were sporadic acts of violence but…

    http://en.wikipedia.org/wiki/History_of_the_Jews_in_England#Stephen_to_Henry_II:_1135.E2.80.931189

    I think it is fair to say that the first systematic persecution of Jews in England was down to Richard I. Certainly the first pogroms directly followed his coronation.
11. mike says:
    June 14, 2011 at 3:32 am

    Where has Ian B gone?

    Can any of you get hold of him to ask if he has any interest in the Bitcoin thread at Brian’s place?
12. NickM says:
    June 14, 2011 at 9:28 am

    mike,
    Buggered if I know. I shall endeavour to perserve.