Yup - just to be clear: the term "naturally aspirated" means that the intake tract is not pressurized but simply uses atmospheric pressure (which is usually around 14.7 psi absolute at sea level) - except of course during a hurricane.
So - a normally aspirated engine sucks air in, mixes it with a bit of fuel (either using a carburetor or fuel injection) and then the mixture is compressed and ignited. If the ambient air pressure is lower - such as at high altitudes - then the engine sucks in less oxygen and therefore it produces less power. Also - if the engine has a carburetor, it will likely be optimised for the air density at sea level - and so at higher altitudes, the mixture will not be correct and that will also impair power delivery. Fuel injection can automatically compensate for altitude changes by measuring the intake air conditions and chemical composition of the exhaust gases. These adjustments can be made continuously many times per second - and that is why FI vehicles always seem to run "just right".
An engine that is not "normally aspirated" is said to employ "forced induction" which means that the intake tract is pressurized by some means. The basic purpose of forced induction is to increase the amount of air forced into the cylinders which can result in a huge power increase for a given engine displacement. The beauty of forced induction is that when you don't need the increased power, the engine reverts to its "true" size and thus in low power conditions can achieve much better fuel consumption than a larger engine operating at a throttled (low power) setting. This particularly useful for engines that have to work over a wide range of different power levels with rapid responses to throttle setting changes - such as cars and motorcycles. Ships and aircraft generally operate continuously at high power levels for long periods of time - so they are quite different from cars and bikes in that respect.
The different forms of forced induction include:
- a blower driven by the engine through either gears or a belt (this is often called a "supercharger");
- a blower driven by a turbine which in-turn is driven by exhaust gases (aka a "turbo-supercharger");
- there is a new set of developments which use a very high speed brushless electric motor to drive the blower (these are also used in the latest fuel cell developments to force hydrogen gas through the FC membrane) - these are becoming known as "e-chargers" or "e-blowers".
Each of these systems has its own set of pluses and minuses. Mechanical supercharger drives tend to be expensive and heavy and can be troublesome, but turbo-driven superchargers can have a significant delay between an increase in throttle position and the actual desired rise in intake boost pressure. This is called "turbo-lag" and most of the turbo-supercharged bikes built in the 1980s by Honda, Kawasaki, Suzuki and Yamaha had lots of turbo-lag, which made them quite difficult to ride smoothly.
The other benefit is that, as noted above, a forced induction engine can produce a great deal more power at a higher altitude where the air is much less dense (and so there is less oxygen available) - which is obviously most important for aircraft. Nearly all WW-II aircraft engines used forced induction. The British, Japanese and German engines normally used mechanically driven superchargers while many US engines used turbo-superchargers (especially the radial engines) which were developed by General Electric in the 1920s. Interestingly, the most successful versions of the famous North American P-51 Mustang fighter used a British-designed Rolls Royce Merlin engine built under license by Packard in Detroit - and it used a mechanically driven supercharger rather than a turbo-supercharger. BTW - most versions of that Merlin supercharger had two stages (i.e. two separate blowers) and also had two different gear ratios in the crankshaft-driven gearbox. This two-stage, two-speed supercharger was a complex piece of gear - but it worked really well and gave planes like the Spitfire, Mosquito and the US Mustang the ability to perform well at a wider range of altitudes. It was fastened to the back of the engine by no fewer than 80 bolts.
Incidentally, the German (and some Italian) engines were nearly all fuel injected while the Allied engines, including the Russians, and the Japanese engines used carburetors. Those carbs were pretty sophisticated though because they could compensate for the huge air density changes from sea level right up to 25-35000 feet where some of these plane operated. Automotive and motorcycle carburetors generally cannot make those types of adjustments while operating.
The other variable is the location of the carburetor or fuel injection nozzle relative to the intake blower. Some systems are "suck-through" wherein the blower is downstream of the carb or FI nozzle and others are "blow-through" where the carb or FI nozzle is sited downstream of the blower.
There are several different types of compressors including centrifugal (the most common - looks like a cooling fan on a weed-whacker motor), Roots, scroll (usually found on larger engines) and very occasionally, an axial compressor which looks much like the big fan on the front of a large aircraft engine. As far as I know, the only modern motorcycle with forced induction from the factory is the Kawasaki Ninja H2 - which has a gear driven two-speed centrifugal supercharger. It is pretty sophisticated and can produce more than 300 HP from a 1 litre engine.
There are other related add-ones including intercoolers which are radiators that cool the compressed intake air (and thus make it more dense) and turbine waste gates which limit the pressure build-up usually to around 6-10 psi, so that the engine doesn't explode.
Finally, any reciprocating piston engine can employ forced induction: a gasoline engine or a diesel, operating on either a four stroke cycle or a two stroke cycle, although few gasoline two strokes are built these days due to emissions concerns (and very few small two stroke gasoline engines have used forced induction). I had a student many years ago who developed an intercooled turbo-supercharged installation for a 650cc two-stroke Skidoo snowmobile engine and it started easily, ran very well and went like an absolute mad fool at Bombardier's test track in Valcourt Quebec just south of Montréal. His work was incredibly well done - so these things are possible. Bombardier was grateful for the work (they paid for all the parts and supplies he used) - but they never marketed the machine because (I think) they felt that it would simply be too much for the consumer market.
Anyhow - all that is aside from the fact that the Honda ST1100 and ST1300 both have naturally aspirated engines - the former with carburetors and the later with closed loop electronic fuel injection.
