Question

Alcohols are unsuitable as diesel engine fuels because

Answer 1

Why alcohol is unsuitable for diesel engine fuel?
Basics first. In diesel engines, self ignition of the fuel happens due to compression. This is due to lower ignition delay, i.e. the time between rise in temperature and commencement of combustion.

This ability for self ignition under compression is better expressed as cetane number. This is where alcohol lacks, having a very low cetane number and hence poor ignition upon compression.

That is your answer about why alcohol is a poor diesel engine fuel.

However, there is a work around this problem. A dual injection engine, wherein normal diesel is first injected, starts combustion, followed by alcohol injection; can use alcohol as fuel.

In fact such an engine will produce lower soot, similar power characteristic at high loads.