On the face of it, the deal announced between the world's leading mainstream car maker, and the world's premium car maker is all logic. Toyota (which has substantially ignored diesels) needs the best engines it can get for the models it makes in Europe; BMW (which doesn't do full hybrids) can see a need for top-class battery know-how, as soon as possible. Each has the capacity to scratch the other's itch.

But look below the surface and there are fascinating undercurrents. German manufacturers, BMW high among them, have always been distinctly sniffy about the Japanese, apart from admiring their superb manufacturing processes and their perfectly controlled body panel gaps. Not now. BMW knows it has created much expectation with its i3 and i8 electric car concepts and has to support them with the world's best battery technology: Toyota is years down the road to the right solutions.

Toyota, meanwhile, has always enjoyed ploughing its own furrow. It has done some co-operative deals, but the Japanese giant was always irrefutably in the lead, with the "helpee" obviously lower in the pecking order. But with hybrids not selling in Europe in the desired numbers, Toyota - and PSA's diesel hybrids looking distinctly threatening - it suddenly perceives the size of the job (and time needed) to provide itself with a range of clean diesels that can match the excellent products of VW, PSA and Ford, to name three. BMW has the best range available.

There are a couple more fascinating sidelights. First, in today's press conference, no-one at BMW said anything about taking an interest in Hybrid Synergy Drive, the Prius propulsion system which other manufacturers admire but most consider too complicated and expensive to emulate. It will be fascinating to see if BMW approves of the Toyota system enough to choose it for its own models.

Second, BMW and Toyota officials made it clear they had been working on this deal for most of the year. Its first obvious benefit (BMW supply of diesels to Toyota) doesn't come until 2014. It seems clear from the tone of today's announcement that the partners' plans for co-operation are to be deep and long-lasting. This could turn into a project far bigger and more important than any of us visualises at present.