And I need you to shed light on that stowaway incident...

Hello Baraza,

Between the Honda Fit Hybrid, Honda Civic and the Mazda Axela, which would you recommend?

It’s a close call between the Civic and the Axela. The Axela may suffer from a now-infamous Mazda reputation of rarity of parts, though the dealer formerly known as CMC keeps contacting me breathily declaring, “We stock Mazda parts. What are these people on about?”, but then again the Axela is that much more fun to drive.

The Civic is supported by a local Honda dealership (I hope) but if not, there is a famous shop along Nairobi’s equally famous Grogan (Kirinyaga) Road that sells parts for all kinds of Hondas, both real and imagined. However, any non-sporting model, that is any Civic that is not the Si or the Type R, is a snooze-fest when palmed considerably, so I guess it boils down to how much you like driving when deciding which of the two vehicles tickles your fancy.

It also comes down to how much you like looking at your car. The Axela is handsome, snazzy even. Some of the Civic designs are either Korean-generic or acquired-taste-wild. I say “some” of the designs because the same generation of Honda Civic can look like several totally different cars depending on the market in which it is sold, and also whether or not it is a Type R. At least the Axela has the decency to look the same anywhere it goes.

The counterpoint is the Civic is marginally cheaper compared to the Axela, but the differences we are talking about here lie in the five figures, or “amounts that can be compensated for with a little spirited haggling”. 2011 Civics are peddling for about 900 flat while Axelas are in the 950-980 range. Of the two cars, I’d still go for the Mazda.

For reasons the Honda Fit Hybrid is not in the running, there are only two: first, it is a Honda Fit and two, it is a hybrid. To understand these two points, please revisit our 2019 archives. There is more than enough evidence in there as to why either of the two qualities may theoretically be good but in practice, may not be that good.

Would you recommend double transmission plugs for a Toyota iST?

Baraza,

Can you put double transmission spark plugs in a Toyota iST 2003 model? Also, what are the advantages of double transmission spark plugs?

Hi Moses,

Yes, you can install such plugs in an iST, but you don’t need to because it’s a waste of money. Only install them in cars that came with them from the factory. The iST is not one of those cars.

[I presume by “double transmission” spark plugs you actually mean twin-electrode affairs.]

The advantages of twin electrode plugs lie in something called flame front propagation, whereby if you light a fire from two different points, that fire will spread faster compared to having a single point of origin. However, this highly depends on the design of the combustion chamber, and given that the twin electrode is made of one centre electrode and two ground electrodes, it only makes sense to use it if the combustion chamber design involves large spaces on each side of the spark plug electrodes.

Some engines have this design, which is why they are manufactured to run more efficiently on twin-spark technology. When the plug fires, the two ground electrodes ignite the intake charge in two different directions for faster flame spread, hence faster combustion, rather than in one direction, which forces a singular flame front to spread in two different directions, which is slower.

However, a lot of engines have angled combustion chamber designs such that the spark plug is located within the side with the largest volume, or the spark plug is located on one side of the combustion chamber. Once the spark ignites, the flame front only has one direction to go.

It therefore makes no sense to have two ground electrodes since the net effect would be exactly the same as having one ground electrode.

(Addendum: some instances of twin-spark technology have the combustion with two spark plugs instead of one. This mostly applies to motorcycles.)

Besides cars, I know quite a bit about planes too …

Dear Baraza,

I need you to shed light on that stowaway incident. From the reports, the stowaway’s body was a block of ice when it fell off the sky and hit the pavement, meaning it had been subjected to extreme cold conditions. In an earlier story about two stowaways who survived, they narrated how the wheels of the plane burnt them as they retracted upon take-off.

My question is on tyre behaviour. When gases are compressed and are subjected to low temperatures, they liquefy. What ensures that the tyre does not burst upon the plane's landing, since the compressed air may have liquefied at the cruising level, noting that on touchdown, extreme hot conditions vaporise the air rapidly, creating a ‘bomb’?

Also, wouldn’t the freezing conditions at the cruising height of a plane cause crumpling of the wheel as contraction of the compressed air takes effect? Finally, is there ample time to allow the wheel to achieve normal size and pressure as the plane approaches landing?

Regards,

R. Mwangi

Well, well, well, this is an odd one. Chemistry, physics and biology all meeting up in a single query, which is about aviation more than it is motoring. Curiouser and curiouser … Today is your lucky day because not only do I consider aviation the kissing cousin of motoring, I am a bit of an enthusiast about the field myself. Let us sweep away the cobwebs of morbidity and reach the silky package at the centre that is Science …

1. Now, normal air is about 78 percent nitrogen and 21 percent oxygen, approximately. Nitrogen has a boiling point of -196 degC, which is so far below naturally occurring temperatures that you will likely never find it in liquid form outside of a laboratory (or a canister that came from a lab).

2. I have flown a lot, meaning that I have had the chance to get bored enough to start fiddling with the onboard features of a passenger aircraft, one of these features being the screen that shows you movies, or various random statistics such as distance to your destination, altitude or if you are lucky enough to board a jet from a high-end national carrier, various camera feeds from different points of the plane exterior. It’s actually very cool.

One of the statistics you will come across on that screen is external temperature, and if I recall right, this temperature usually varies between -60 degC and -40 degC, depending mostly on the cruising altitude, which for most transcontinental jets, is between 30,000 feet and 40,000 feet … or 12km up in the air, in Protestant units.

So, as you can see, the ambient temperature at the cruising altitude of a typical commercial aircraft is not low enough to liquefy nitrogen. So, to answer your first question: the air never liquefies even in flight, because the prevailing temperatures never dip low enough to the point nitrogen liquefies.

Nitrogen

Let us talk a little about the R35 Nissan GTR, Godzilla. It came out 13 years ago (good gracious, talk about long-in-the-tooth) to great acclaim because of the kind of onboard technology it was packing and the kind of obsessive and fanatical methods Nissan used to build the car. One of these insane techniques was the decision to fill the tyres with pure nitrogen instead of normal air. Why? Go back to point 1 above: normal air is 78 per cent nitrogen, 21 per cent oxygen while the remaining 1 per cent is a mix of other nebulous vapours, including water.

Nitrogen is a very stable gas, meaning it’s physically and chemically lazy. Unlike the other gases making up the 22 percent balance of atmospheric air, nitrogen won’t boil, won’t freeze, won’t react and is not very susceptible to temperature and pressure changes unless the gas is subjected to extremes.

This is what Nissan was thinking about when they said the tyres in the R35 should have pure nitrogen in them instead of regular old 78 per cent-nitrogen atmospheric air. Nissan was exercising such an unhealthy amount of control over the build and dynamics of the GTR that they did not want tyres that behaved unpredictably (this was purely theoretical). A mix of gases is unpredictable in behaviour, but a single gas isn’t. So single gas it is.

(The fastidiousness was not only obsessive, some of it, such as using nitrogen in the tyres, was bit unnecessary. The car is built in a hermetically sealed environment to eliminate the microscopic expansion and contraction of materials caused by mild fluctuations in room temperature, and the suspension is developed under a rig that simulates the weight of the vehicle so that when it is finally installed, no further tuning is necessary since all the parameters have been nailed the first time round. Nobody builds a car like this: it is unsustainable, just like filling a road car’s tyres with nitrogen when not even Formula One goes to such insane extents.)

It might border on the pathological when you choose to fill your tyres with nitrogen and you live, say, in areas of high climatic certainty such as the tropics, but when it comes to aircraft, things are different and a lot more serious.

Plane tyres are filled with nitrogen for the same reasons Nissan claimed with the GTR and the logic behind it is the root of your inquisition: the likelihood of temperature and pressure fluctuations playing havoc on the gas and causing a massive crash during take-off or landing.

The difference is: Nissan filled their tyres with nitrogen to micromanage the GTR’s handling and try to beat the Porsche 911 in an all-out Nürburgring battle (which they ultimately lost, by the way, in no small part because the GTR is 13 years old). Airlines fill their tyres with nitrogen because if they didn’t, their planes will suffer tyre failures and crash on landing. One of these groups of people has a good reason to play around with nitrogen. The other one is just being petty.

Rubber

Aircraft tyres and car tyres are broadly similar but aircraft tyres are a lot more robust for obvious reasons: they support a much larger tonnage, handle much higher velocities and undergo vastly increased braking stresses.

To meet these stressful conditions, aircraft tyres are made from rubber, nylon, cord and steel, which are combined in a process called vulcanisation that toughens up the resultant blend considerably.

The tyre compound is so tough that it resists the change in volume and physical degradation that come from temperature fluctuations by sheer stubbornness. However, besides the sheer resilience of the tyre compound, the main reason the tyres don’t burst even after repeated landings of 50-tonne vessels at speeds north of 260km/h is something we have already mentioned, at the top of this response: air.

The airplane tyres are inflated to pressures six times higher than those of car tyres, about 200psi, but they are capable of withstanding up to 800psi before bursting. Most of you do 30 psi in your car tyres.

That highly pressurised air provides a veritable cushion that not only absorbs shocks from repeated landings, it also further strengthens what we have already agreed is a very tough tyre to start with; in fact, most engineers will credit the tyre pressures more than the toughness of the tyre itself as to the reason things don’t fall apart the moment an aircraft touches the ground.

The morbid truth

About those poor stowaways, the explanations are very simple. The poor sod who was transformed into an icy stalactite did so because the human body is about 60 per cent water, with the lungs being 83 per cent water and the heart and brain being 73 per cent water. Water freezes at 0 degC and as I said, ambient temperature at cruising altitude is between -60 degC and -40 degC.

That is low enough to freeze the water in the human body into rock-hard ice within seconds, hence … yeah.

The burning is also easy to explain away. Tyres have something called rolling resistance, and the reason they have grip is friction. Both rolling resistance and friction cause the tyres to heat up.

Now, picture a tyre that has to support 200 tonnes or more, and still attain take-off speeds of up to 280km/h.

I haven’t heard of an aircraft with a tyre temperature readout, most have brake temperature sensors instead, but it doesn’t take a rocket scientist (haha) to figure out that the tyres will heat up on take-off, and heat up enough to cause serious burns to the human body if it comes in contact with them.