I'm struggling to understand why, apart from viscosity, it's so easy to substitute lipids (i.e. SVO, WVO) for diesel fuel. I'd have to assume these fuels have inherently different octane ratings and thus require different heat and pressure values to combust. How is interchangeability not an issue when every fuel is expected to ignite at exactly the pressure the chamber reaches right before TDC and not any degree before or after?
I believe you have a fundamental flaw in your thinking. The combustion event is controlled by when it is (direct) injected into the combustion chamber. This controls the timing of the event, not the cetane rating or any other factor of the fuel itself. There are other factors which control the burn rate, such as: shape of the piston; shape of the combustion chamber; amount of fuel injected; compression ratio; engine speed; (among others). This is the reason why the fuel can be interchanged without worrying about the fuel ignition point itself.
EDIT: To answer some of your lingering questions in the comments:
If combustion is timed by point of injection then there must be sufficient pressure in the chamber at that moment. If that is the case wouldn't the pressure in the injectors have to be higher to get the fuel into the chamber?
Yes, it is much higher than the pressure in the chamber at the time of the event. Some injection systems do their thing at around 44,000 psi. There is a lot of pressure built during the compression phase of a diesel engine. This compression creates a lot of heat and pressure. There needs to be sufficient pressure in the injection system to overcome the cylinder pressure.
Why doesn't the fuel ignite in the higher pressure injection system? Higher heat in the chamber maybe?
Because there isn't any oxygen in the lines to promote combustion.
What about IDI's, do they operate in the same way?
They do not operate in the same way. They have what is called a pre-chamber where mixing of fuel and air occur. The fuel is injected into the pre-chamber at a much lower pressure. The combustion event is controlled completely by when the compression produces enough heat to ignite the fuel. Then the combusting fuel/air is thrown into a portion of the piston, where it ignite further and completes the burn process. The shape of the piston and pre-chamber are the parts which control the burn itself. If not for these, this engine wouldn't work efficiently enough to work as an engine. This Wikipedia article gives the pros/cons to IDI:
- Smaller diesels can be produced.
- The injection pressure required is low, so the injector is cheaper to produce.
- The injection direction is of less importance.
- Indirect injection is much simpler to design and manufacture; less injector development is required and the injection pressures are low (1500 psi/100 bar versus 5000 psi/345 bar and higher for direct injection)
- The lower stresses that indirect injection imposes on internal components mean that it is possible to produce petrol and indirect injection diesel versions of the same basic engine. At best such types differ only in the cylinder head and the need to fit a distributor and spark plugs in the petrol version whilst fitting an injection pump and injectors to the diesel. Examples include the BMC A-Series and B-Series engines and the Land Rover 2.25/2.5-litre 4-cylinder types. Such designs allow petrol and diesel versions of the same vehicle to be built with minimal design changes between them.
- Higher engine speeds can be reached, since burning continues in the prechamber.
- Specific fuel consumption is higher than with direct injection because of heat loss due to large exposed areas and pressure loss due to air motion through the throats. This is somewhat offset due to indirect injection having a much higher compression ratio and typically having no emissions equipment.
- Glowplugs are needed for a cold engine start.
- Because the heat and pressure of combustion is applied to one specific point on the piston as it exits the precombustion chamber or swirl chamber, such engines are less suited to high specific power outputs (such as turbocharging or tuning) than direct injection diesels. The increased temperature and pressure on one part of the piston crown causes uneven expansion which can lead to cracking, distortion or other damage due to improper use; use of "starting fluid" (ether) is not recommended in glow plug, indirect injection systems, because explosive knock can occur, causing engine damage.
NOTE: Some of the pro's for the Wiki article are fairly moot in this day/age and pertain mainly to why it was used in the past during the 70/80's. Compact direct injected diesel engines are quite commonplace now.