Spark Plugs in Diesel Engines; possible?

To start with, it's not about what's possible and what's not, Engineering doesn't work that way. The quest of what's possible is what scientists embark onto but Engineers use their limited neural resources to make certain things work in the best way possible. So, why don't we use Spark plugs in Diesel Engines?

The answer is very simple. There are two primary reasons due to which Spark Plugs cannot be used to ignite diesel fuel in diesel engines.

1. Diesel has low volatility and thus it cannot form desired air-fuel mixture to be ignited by spark plugs.

2. Secondly, diesel has a lower self-ignition temperature than petrol/gasoline. The temperature generated by compression of air during compression stroke, which is around 1000^F (538^C) or higher, is enough to ignite the atomized diesel fuel (sprayed into the hot-compressed-air at the end of compression stroke).

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Let's clear some ambiguities!

First of all, many people point out that Diesel engines have higher compression ratio, which helps air to achieve higher temperatures and pressure, so they don’t need a spark plug. I would like to revert this statement so that it’s taken up correctly by the readers.

Diesel engines are designed to have a higher compression ratio. And, they are essentially an improvement over Petrol engines.

Diesel engines are DESIGNED to have higher compression ratios, keeping in mind that diesel is less volatile and has low self-ignition temperature. It's this fact which makes them more efficient than Petrol engines. The other potential reason is their power cycle.

Moreover, Diesel engines are an improvement over Petrol engines (Otto-cycle engines). Nikolaus A. Otto built a four-stroke Spark Ignition (Petrol) engine in 1876 using the cycle proposed by Alphouse de Rochas (called Otto cycle). The major shortcoming for these engines is that they can’t operate at high compression ratios, because if they did, air-fuel mixture may pre-ignite, or may lead to detonation and knocking, later in the combustion cycle. On the other hand, the Compression Ignition (Diesel) engine was proposed by Rudolf Diesel in 1890s. We know as a fact that, in Compression Ignition engines, only air is compressed during the compression stroke. Therefore, diesel engines can operate at much higher compression ratios.