Beyond @Francis_Vaughan’s excellent points, thrust asymmetry in a failure is not that big a deal. You make the vertical fin/ rudder a bit bigger and you’re done. Conversely, if you switch to tail-mounted engines you could shrink the tail & save the weight and drag.
Except … Now the thrust reversers disturb the airflow over the tail and the fin/rudder is ineffective at controlling your path during rejected takeoffs and ordinary landings. The MD-80 suffered grievously from that. Compared to the DC-9 the engines were much bigger with much more reverse thrust in a larger plume. But the tail was the same size. There were lots of restrictions on limiting reverse thrust lest the airplane get squirrelly on landing. Every MD-80 driver, including me, has felt the damn thing start going sideways after you overdid the reverse a bit in a crosswind.
Rear-mounted engines also pretty well require a T-tail or at least a cruciform tail. The rudder/fin structure required to carry the horizontal stab/elevator loads to the fuselage is a LOT heftier than you might expect. The inside of the vertical fin on a 727 or MD-80 looks like it belongs in a railroad bridge, not a flying machine. The 737’s fin innards look positively delicate by comparison.
There are more factors to the decision, but I’ll quit now.
Bottom line: tail-mounted engines are no free lunch.
I just bought a new Ram pickup with the 6.4L gas engine. It has two plugs per cylinder.
In keeping with the thread subject, it has 410 hp. My 2000 model pickup with 7.3L had 295 hp.
In the 70s I was a part-time mechanic (at a filling station). I’m familiar with ICEs and have rebuilt a few. But I literally cannot identify many of the objects, mysterious piping and one entire assembly (the size of a breadmaker) in the new truck. It’s demoralizing that I have no idea what these things are.
Thanks to @LSLGuy for filling in the details. It is 40 year old technology, (Yipes! indeed) so it may have taken me a while to find references. As ancient as the technology seems it arose in the age of distributor ignitions on car engines. There’s a large window of time available for the spark compared to how long it takes for complete fuel combustion and the multiple sparking could insure complete combustion and overcome problems with water in the fuel or air that would slow ignition. Small planes had their own water problems with ice accumulation in common carburetors. I don’t know if these ignitions were accepted as safe in airplanes, it was argued that they were more reliable at keeping an engine running than the brief spark provided magneto ignitions. Similar considerations applied to the use of electromechanical fuel injectors for a diesel aircraft engine. They were considered less reliable than mechanical injectors activated by a cam shaft connected to the engine crankshaft. The new design used independent injectors that alternated on each cycle. Some failures could be detected and the remaining injector could be run on every cycle. If failures went undetected the engine would keep running even if fuel was injected only every other cycle.
Keep in mind that when a plane is flying, it’s the wings that are holding up the fuselage, not the other way around. Moving heavy stuff, like fuel and engines, to the wing means that the spar is carrying a lighter fuselage.
Thanks for all the info. Obviously, there are second and third order engineering considerations that begin to matter at extremes of weight, speed, etc.
Just as an interesting point, I many years ago bought an old VW Beetle (my first car) and adventurously tried to do my own tune-up. I inadvertently swapped the spark plug leads on one side, and the car ran fine until it approached 30mph and began to get rough - giving you an idea how forgiving the spark timing could be and still work.
Whereas I was told (never verified) that my early Honda Civic simply doubled up the sparks - fired 2 of 4 plugs at a time, simplifying the electronics; since one of those plugs was on the exhaust cycle and a spark had no effect.
I had a 4-cylinder 125hp outboard motor that worked the same way. Two coils, each attached to two plugs. When the coil was triggered, the two plugs in separate cylinders fired. The one plug did something useful and the other one did nothing in the middle of exhausting.
My first BMW motorcycle (a '99 model with a boxer twin engine) had the spark plugs for both cylinders on the same coil and fired them on every crank revolution. It also fired both fuel injectors on every crank revolution. This alleviated any need for a cam position sensor.
But that sounds like they’d be pissing away 50% of all the fuel. Giving half the fuel mileage and lots more than double the air pollution. That seems … un-German.
Since any given cylinder’s fuel injector squirts twice for each complete four -stroke cycle, that just means each injection event needs to supply half of the required fuel for that cycle. These are port fuel injectors, so no fuel gets into the combustion chamber until the intake valve opens and the intake stroke of the piston happens. Maybe you were thinking of direct fuel injection (in which the fuel injectors fired directly into the combustion chamber)?
Well, that might be true if the cost of catalysts for hydrogen fuel cells goes down and the production and logistics issues of hydrogen fuel are resolved. I am not convinced that this will ever be the case without substantial improvements in both catalysts and storage density of hydrogen.
PGMs are a minor contributor to the cost of a PEMFC system, which is dominated by balance-of-plant components like electronics and plumbing. And catalyst loadings keep decreasing.
Toyota is coming out with a hydrogen-powered car. But it doesn’t use fuel cells. It uses a hydrogen internal combustion engine. I didn’t know it was possible to make an ICE that burns hydrogen.
But is that infrastructure needed? Mass production of EV’s means even 3rd world countries will be buying used EV’s, or at least electric drivetrains eventually and will make them work as that will be all that is available. Even if this means putting a generator on the roof, or perhaps recharging with used solar panels.
As for supply limitations, we are just starting to mine lithium, not too long ago no one did, but lithium was just a byproduct of other mining. Now that the free markets are actually looking for it it seems like it will be found. So what seems somewhat rare today might not in a decade.
Burning hydrogen is by far the easiest thing to do with it technologically. Much easier than, e.g. running a fuel cell with it. At least from where our deployed tech is today. If an ICE can run on gasoline or propane or methane (AKA “natural gas”) with comparatively minor tweaks, it can certainly be so tweaked to run on hydrogen. The same is true of gas turbines used in some ships and all modern airplanes.
As with propane/methane, the practical challenge is not in building an engine to burn it. It’s in designing and deploying the infrastructure so millions of those engines can be supplied with fuel every day. But most of those challenges aren’t technological engineering. They’re cost-effectivity engineering and financial engineering and legal engineering. Which are often the three hardest kinds.
Hydrogen has a really high octane rating, so you can design the engine to operate at a higher compression ratio (and therefore a higher efficiency) than is typical for gasoline engines.
Using hydrogen obviously means no carbon. Whether there’s no CO2 associated with operating this engine will depend on how the hydrogen fuel is produced, but you definitely won’t get any CO2, CO, raw hydrocarbon, or particulate matter out of the tail pipe. (Ok, maybe you’ll get a smidge just from crankcase oil that gets burned, but it won’t be much.)
OTOH, the laminar flame speed for hydrogen is many times that of gasoline (about 3 m/s, versus about 0.4 m/s for gasoline). This has consequences for the rate of pressure rise during the combustion event: high rates of rise manifest as audible roughness, the kind of sound you hear from a diesel, which a lot of drivers don’t care for.
There’s also the question of range. Since you can’t use liquified H2, it’s hard to store enough fuel onboard to go very far, unless you are willing to use a hybrid drivetrain to maximize fuel economy. But the article you linked to says that Toyota is specifically avoiding going electric on their H2 Corolla, so I wonder what kind of range it has.
That was my guess too - specifically considerably higher compression ratios, along with variable ignition timing and computer-controlled fuel injection.
It seemed to me having driven some mid-late 1970s and early 80s junkers, that power really improved once fuel injection became normal, and along with it computer control of the engine.
US manufacturers could make more power- the original Chevy 350 as used in the 1969 Corvette had a 11:1 compression ratio and used high octane gas, to give 350 horsepower. But, like others have pointed out, the emissions regulations in the 1970s crippled the power output of the older design engines, so there was a period of really anemic engines until the mid-late 1980s, when fuel injection and computer control came into production and power jumped back up.
Since then, it’s been incremental improvements- variable valve timing, four valves per cylinder, direct injection, and so on.