What are the alternatives?
Auxiliary motors are powered typically either by stored electricity or petrol.
Although huge advances have been made in the storage of electricity in batteries, battery weight and recharge time is such that electric-assisted bikes tend to be limited to short distance, limited power inner city environments. Also, batteries are expensive. If your travel circumstances are suitable, then they might be OK for you.
Petrol-assisted motorized bicycles tend to fit into 3 weight-related categories:
- Light motor-assisted bicycles (up to 27 kg)
- Common (heavy) motorized bikes (30 to 50 kg)
- Mopeds (100 kg)
Of the 3 categories above, only the light motor-assisted bicycle is a serious contender for use as a conventional bicycle. Weight is a primary consideration.
Heavy motorized bikes are usually fitted with 4-stroke motors. They are bulkier and tend to include gear boxes and engine starters and/or batteries. Increasing complexity means that cost is higher than for the light motorized bikes.
These days, mopeds tend to be light motorcycles with the same capability of motorcycles, but with limited power/speed capability.
Energy conservation is a key objective for a serious cyclist. Therefore weight and resistance minimisation are key aims.
In many cases, once a motor has been added to a bicycle, the additional weight prevents it from being used as a conventional bicycle - derailleur gears, standard componentry and standard wheels. These days, 15 kg is a heavy commuter bicycle. Add another 30-40 kg - motor, gear box, starter, heavy duty brakes etc and it is not a bicycle any more. It has become a scooter, and small scooters typically weigh in at 100 kg.
The common internal combustion engine (ICE) motor - chain drive bicycle often has the motor mounted in the front triangle of the bicycle frame with a chain drive to a sprocket fixed to the spokes. The direct chain drive between the back wheel and motor makes the motor very easy to start by simply pedalling off from the stationary position and dropping the clutch.
This is a short essay covering the sort of issues that affect your personal transport decision-making. It should only be used to kick-start your own research on the subject as it has been no expert review of the essay.
Emissions from ICE motors
Commonly available motor technologies for bicycles are 2-stroke and 4-stroke motors. Both emit pollution, predominantly burnt hydrocarbons. Old 2-stroke motors also emit a high proportion of unburnt hydrocarbons.
Unburnt fuel emissions are a locality-specific problem. They do not accumulate in the atmosphere. Air quality professionals talk about them as NMVOC (non-methane volatile organic compounds). Methane is a significant greenhouse volatile organic compound but not a by-product of dirty gasoline motors. Unburnt hydrocarbons can be a serious health problem in cities, but not in sparsely populated areas such as Australia. Pollution from unburnt hydrocarbons does not accumulate in the atmosphere.
Greenhouse gas mainly in the form of carbon dioxide is the product of burning fossil fuel. Greenhouse gases accumulate in the atmosphere and most scientists agree that the planet does have a serious greenhouse problem due to fossil fuel use. Greenhouse gases cause climate change, which in turn stresses the environment.
The source of power for most electric-assisted bicycles is the product of burning coal. Therefore, electric-assisted bicycles are not necessarily more climate-friendly than petrol-assisted bicycles.
2-stoke vs 4-stroke
2-stroke motors have been responsible for air pollution in cities because they characteristically emit a high proportion of unburnt hydrocarbons. 2-stroke motors have a higher power-to-weight ratio compared with 4-stroke motors and are simpler and therefore cheaper than 4-stroke motors.
It is interesting to note that, in Australia, 2-stroke motors have never been required to meet an emission control standard, but international regulations have seen their numbers dwindle.
On the basis that 2 strokes could be used sparingly in conjunction with pedal power and regulated in such a way that they could be excluded from problem areas, then 2-stoke pollution harm could be negligible.
Furthermore, modern Direct Injection technology has seen the emergence of clean 2-stroke engines, especially in the form of 2-stroke outboard motors. Also, read Envirofit's 2-stroke retrofit article  using technology developed by Australian engine technology company Orbital . It is quite feasible that dedicated bicycle engines, both very clean and very light weight, but they are not yet readily available.
In terms of climate impact, ie burnt hydrocarbons, the 2-stroke is much the same as a 4-stroke, kilowatt for kilowatt. But the humble 2-stroke has a distinct advantage, it is a lighter weight. A 4-stroke engine of equivalent power is about twice the weight of its 2-stroke equivalent. If the lighter weight means that its use can actually be reduced, then the overall carbon footprint could be much lower, in terms of greenhouse gas emissions per kilometre.
The energy density of gasoline is about 47 megajoules per kilogram. The energy density of lithium-ion batteries is about 0.54 - 0.72 megajoules per kilogram, lets say about 0.6 megajoules per kilogram for an older battery. Even if a dirty 2-stroke is only yielding 30 megajoules per kilogram, then energy per kilogram is still 50 times that of the battery. 0.5 litres of gasoline equates to more than 25 kilograms of battery.
You can manhandle 25 kilograms of bicycle, but you can't manhandle 50 kilograms of bicycle. You can lift a 25 kilogram bicycle fitted with a 2-stroke motor onto a vertical bicycle storage rack, but not a 50 kg bike unless you are an Olympic weightlifter. E-bikes tend to be underpowered and are best suited to short haul commuters. In anything else they are power-deficient.