I want to mention that the Nicad voltage vs the dry cell voltage isn't the entire story.
A battery also as an internal resistance. If you put a no-load voltmeter on a freshly charged nicad and new dry cell, you will see this voltage, and it would seem like the nicad suffers in comparison. However if you hook up either battery to a load (like a glow plug), the current (which after all is what is heating up the glow plug) that flows depends not only on the voltage, but upon the resistance not only of the glow plug, but also the batttery, with current= Voltage/(Rbat+Rglow). For the same size cell, the nicad typically has much less internal resistance, and that is why they work so well for us. If you hook up 2 alkaline D cells in parallel, you can halve the effective battery resistance, and then the dry cell does a pretty good job (that's what I use). Using one of the big dry cells is equivalent to lowering the internal resistance (which is depending inversely on the area between the positive and negative foils of the battery). Of course in the end, you can recharge your nicad, but have to buy a new drycell.